Delete not user method for plugin_generator

[soartec-lab]

Summary

In the following #35894, this method is forgot to delete, so I deleted it.

Other Information

There is no difference in the generated file.

Before

gem 'rails', '~> 6.0.0.beta3'

$ bin/rails plugin new blorgh --mountable
create
create  README.md
create  Rakefile
create  blorgh.gemspec
create  MIT-LICENSE
create  .gitignore
create  Gemfile
create  app
create  app/controllers/blorgh/application_controller.rb
create  app/helpers/blorgh/application_helper.rb
create  app/jobs/blorgh/application_job.rb
create  app/mailers/blorgh/application_mailer.rb
create  app/models/blorgh/application_record.rb
create  app/views/layouts/blorgh/application.html.erb
create  app/assets/images/blorgh
create  app/assets/images/blorgh/.keep
create  config/routes.rb
create  lib/blorgh.rb
create  lib/tasks/blorgh_tasks.rake
create  lib/blorgh/version.rb
create  lib/blorgh/engine.rb
create  app/assets/config/blorgh_manifest.js
create  app/assets/stylesheets/blorgh/application.css
create  bin/rails
create  test/test_helper.rb
create  test/blorgh_test.rb
append  Rakefile
create  test/integration/navigation_test.rb
vendor_app  test/dummy

After

gem 'rails', git: 'https://github.com/soartec-lab/rails', branch: 'delete_not_use_method'

$ cat Gemfile.lock
GIT
  remote: https://github.com/soartec-lab/rails
  revision: 831fa3dacb3eb1f20335748059d0e0a10dbb2b78
  branch: delete_not_use_method
・
・
・

$ bin/rails plugin new blorgh --mountable
create  
create  README.md
create  Rakefile
create  blorgh.gemspec
create  MIT-LICENSE
create  .gitignore
create  Gemfile
create  app
create  app/controllers/blorgh/application_controller.rb
create  app/helpers/blorgh/application_helper.rb
create  app/jobs/blorgh/application_job.rb
create  app/mailers/blorgh/application_mailer.rb
create  app/models/blorgh/application_record.rb
create  app/views/layouts/blorgh/application.html.erb
create  app/assets/images/blorgh
create  app/assets/images/blorgh/.keep
create  config/routes.rb
create  lib/blorgh.rb
create  lib/tasks/blorgh_tasks.rake
create  lib/blorgh/version.rb
create  lib/blorgh/engine.rb
create  app/assets/config/blorgh_manifest.js
create  app/assets/stylesheets/blorgh/application.css
create  bin/rails
create  test/test_helper.rb
create  test/blorgh_test.rb
append  Rakefile
create  test/integration/navigation_test.rb
vendor_app  test/dummy

[soartec-lab]

Thank you merge !