Remove usages of self-monitoring related code in backend

Relevant files

• app/controllers/admin/application_settings_controller.rb
• app/workers/all_queues.yml
• app/services/metrics/dashboard/clone_dashboard_service.rb
• lib/gitlab/metrics/dashboard/finder.rb
• lib/gitlab/metrics/dashboard/service_selector.rb
• spec/lib/gitlab/metrics/dashboard/service_selector_spec.rb
• spec/services/metrics/dashboard/clone_dashboard_service_spec.rb
• spec/support/helpers/metrics_dashboard_helpers.rb
• [TBD] any files that remain for usage removals