Remove KindleGen installation

[yahonda]

This pull request removes KindleGen installation KindleGen is not available for download.

https://www.amazon.com/gp/feature.html?docId=1000765211

KindleGen is no longer available for download. Please use Kindle Previewer to convert, preview, and validate your eBooks. Kindle Previewer provides the same functionality of KindleGen and, in addition, provides:

Fix #177

[fxn]

Does that Kindle Previewer support some kind of batch processing?

[yahonda]

I've found "Kindle Previewer" is not available for Linux, available for Windows and macOS as described below.

https://www.amazon.com/gp/feature.html?ie=UTF8&docId=1000765261

System Requirements
• Windows 8.1 or later
• Intel Mac OSX 10.13 or later

[yahonda]

Rails supports to generate the guide in EPUB since rails/rails#45539 and deprecates guides:generate:kindle task. So I think we can safely remove KindleGen installation.

After #202 is merged or installing pkg-config and libyaml-dev packages manually, rails-dev-box can generate the epub format guides follows.

• Steps to reproduce

- At Vagrant host
git clone https://github.com/rails/rails-dev-box
vagrant up
vagrant ssh
- At rails-dev-box
git clone https://github.com/rails/rails
cd rails
bundle install
cd guides/
bundle exec rake guides:generate:epub

• Result

$ bundle exec rake guides:generate:epub
/usr/bin/ruby3.0 -Eutf-8:utf-8 rails_guides.rb
Generating security.md as security.html
Generating active_record_encryption.md as active_record_encryption.html
Generating active_support_core_extensions.md as active_support_core_extensions.html
Generating active_record_migrations.md as active_record_migrations.html
Generating index.html.erb as index.html
Generating generators.md as generators.html
Generating action_controller_overview.md as action_controller_overview.html
Generating action_mailbox_basics.md as action_mailbox_basics.html
Generating command_line.md as command_line.html
Generating rails_application_templates.md as rails_application_templates.html
Generating active_record_multiple_databases.md as active_record_multiple_databases.html
Generating action_text_overview.md as action_text_overview.html
Generating 4_1_release_notes.md as 4_1_release_notes.html
Generating 5_2_release_notes.md as 5_2_release_notes.html
Generating routing.md as routing.html
Generating active_storage_overview.md as active_storage_overview.html
Generating association_basics.md as association_basics.html
Generating asset_pipeline.md as asset_pipeline.html
Generating api_app.md as api_app.html
Generating webpacker.md as webpacker.html
Generating active_job_basics.md as active_job_basics.html
Generating active_record_basics.md as active_record_basics.html
Generating plugins.md as plugins.html
Generating 4_0_release_notes.md as 4_0_release_notes.html
Generating 2_2_release_notes.md as 2_2_release_notes.html
Generating 7_1_release_notes.md as 7_1_release_notes.html
Generating active_record_querying.md as active_record_querying.html
Generating i18n.md as i18n.html
Generating action_mailer_basics.md as action_mailer_basics.html
Generating active_record_postgresql.md as active_record_postgresql.html
Generating 3_0_release_notes.md as 3_0_release_notes.html
Generating 7_0_release_notes.md as 7_0_release_notes.html
Generating action_view_helpers.md as action_view_helpers.html
Generating maintenance_policy.md as maintenance_policy.html
Generating configuring.md as configuring.html
Generating autoloading_and_reloading_constants.md as autoloading_and_reloading_constants.html
Generating error_reporting.md as error_reporting.html
Generating _welcome.html.erb as _welcome.html
Generating working_with_javascript_in_rails.md as working_with_javascript_in_rails.html
Generating 2_3_release_notes.md as 2_3_release_notes.html
Generating layouts_and_rendering.md as layouts_and_rendering.html
Generating 6_0_release_notes.md as 6_0_release_notes.html
Generating contributing_to_ruby_on_rails.md as contributing_to_ruby_on_rails.html
Generating 3_1_release_notes.md as 3_1_release_notes.html
Generating debugging_rails_applications.md as debugging_rails_applications.html
Generating 6_1_release_notes.md as 6_1_release_notes.html
Generating getting_started.md as getting_started.html
Generating action_view_overview.md as action_view_overview.html
Generating api_documentation_guidelines.md as api_documentation_guidelines.html
Generating engines.md as engines.html
Generating 5_0_release_notes.md as 5_0_release_notes.html
Generating rails_on_rack.md as rails_on_rack.html
Generating testing.md as testing.html
Generating classic_to_zeitwerk_howto.md as classic_to_zeitwerk_howto.html
Generating active_support_instrumentation.md as active_support_instrumentation.html
Generating 4_2_release_notes.md as 4_2_release_notes.html
Generating initialization.md as initialization.html
Generating form_helpers.md as form_helpers.html
Generating active_model_basics.md as active_model_basics.html
Generating layout.html.erb as layout.html
Generating development_dependencies_install.md as development_dependencies_install.html
Generating upgrading_ruby_on_rails.md as upgrading_ruby_on_rails.html
Generating 5_1_release_notes.md as 5_1_release_notes.html
Generating action_cable_overview.md as action_cable_overview.html
Generating 3_2_release_notes.md as 3_2_release_notes.html
Generating threading_and_code_execution.md as threading_and_code_execution.html
Generating active_record_validations.md as active_record_validations.html
Generating ruby_on_rails_guides_guidelines.md as ruby_on_rails_guides_guidelines.html
Generating caching_with_rails.md as caching_with_rails.html
Generating _license.html.erb as _license.html
Generating active_record_callbacks.md as active_record_callbacks.html
Generating epub/welcome.html.erb as epub/welcome.html
Generating epub/rails_guides.opf.erb as epub/rails_guides.opf
Generating epub/toc.ncx.erb as epub/toc.ncx
Generating epub/copyright.html.erb as epub/copyright.html
Generating epub/toc.html.erb as epub/toc.html
Generating epub/layout.html.erb as epub/layout.html
=> Using book dir: /home/vagrant/rails/guides/output/epub/OEBPS
=> Fixing filenames in Table of Contents
=> Using output dir: /home/vagrant/rails/guides/output/epub
=> Generating meta files
=> Generating EPUB
=> Done Generating EPUB
Epub generated at: output/epub/ruby_on_rails_guides_c819075.epub
$