Remove setup notices

[yscik]

Fixes #3046

Changes proposed in this Pull Request

• Remove Install Sensei LMS Pages notice and related handlers, success notices.
• Remove Enable Usage Tracking notice, dialog, handlers and tests.
• Deprecate public methods related to those

Testing instructions

• Reset a site
• Activate Sensei LMS plugin
• No notices other than the Setup Wizard one should be visible.

Deprecated Code

• Sensei_Admin::install_pages_output

• Sensei_Admin::admin_install_notice

• Sensei_Admin::admin_installed_notice

• Sensei_Admin::admin_notices_styles

• Sensei_Admin::install_pages

• Sensei_Usage_Tracking_Base::hide_tracking_opt_in

• Sensei_Usage_Tracking_Base::is_opt_in_hidden

• Sensei_Usage_Tracking_Base::opt_in_dialog_text_allowed_html

• Sensei_Usage_Tracking_Base::maybe_display_tracking_opt_in

• Sensei_Usage_Tracking_Base::handle_tracking_opt_in

• Sensei_Usage_Tracking_Base::enqueue_script_deps

• Sensei_Usage_Tracking_Base::output_opt_in_js

[renatho]

Cool! A lot of code removed! \o/

Besides the few comments, only a reminder to add the deprecated methods to the removal issue ;)