Remove code and documentation for deprecated Phabricator task importer

For reasons and general context see https://gitlab.com/gitlab-com/Product/-/issues/4894+.

Dev tasks

• Delete importer code - !117649 (merged)
• Remove importer from UI - !117649 (merged)
• Remove routes - !117649 (merged)
• Remove controllers - !117649 (merged)
• Remove importer from Admin -> Settings -> General -> Visibility and access controls -> Import Sources - !117649 (merged)
• Create data migration to remove the importer from the application_settings table. (Similar to !106466 (merged)) - !118026 (merged)
• Remove before_validation from ApplicationSetting model after data migration has been run. This will be done in 16.1 (#408808 (closed))