Remove 'delayed_repository_update_mirror_worker' feature flag

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/362894

Use `sticky` data consistency for RepositoryUpdateMirrorWorker by
default.

Changelog: changed

config/feature_flags/development/delayed_repository_update_mirror_worker.yml

----
-name: delayed_repository_update_mirror_worker
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/87995
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/362894
-milestone: '15.1'
-type: development
-group: group::source code
-default_enabled: false

ee/app/workers/repository_update_mirror_worker.rb

@@ -6,7 +6,7 @@ class RepositoryUpdateMirrorWorker
   include ApplicationWorker
 
   idempotent!
-  data_consistency :sticky, feature_flag: :delayed_repository_update_mirror_worker
+  data_consistency :sticky
   include ProjectStartImport
 
   feature_category :source_code_management

ee/spec/workers/repository_update_mirror_worker_spec.rb

@@ -16,7 +16,6 @@
 
     it_behaves_like 'worker with data consistency',
                     described_class,
-                    feature_flag: :delayed_repository_update_mirror_worker,
                     data_consistency: :sticky
 
     it 'sets status as finished when update mirror service executes successfully' do