Remove code and documentation for deprecated gitlab.com importer

For reasons and general context, see https://gitlab.com/gitlab-com/Product/-/issues/4895+.

Dev tasks

• Delete importer code

• Remove importer from UI

• Remove routes

• Remove controllers

• Create data migration to remove the importer from the application_settings table. Similar to !106466 (merged) - make sure the migration properly serializes to yaml see !118757 (merged)

• Remove importer from Admin -> Settings -> General -> Visibility and access controls -> Import Sources

• Remove before_validation from ApplicationSetting model after data migration has been run. This will be done in 16.1 (#408808 (closed))

• Update description in Admin -> Settings -> General -> Visibility and access controls -> Import Sources.

  From: Code can be imported from enabled sources during project creation. OmniAuth must be configured for GitHub, Bitbucket and GitLab.com
  
  To: Code can be imported from enabled sources during project creation. OmniAuth must be configured for GitHub and Bitbucket