Remove load_balancing_for_update_all_mirrors_worker FF

The feature flag has been enabled for a while, we can remove it

Changelog: changed

config/feature_flags/development/load_balancing_for_update_all_mirrors_worker.yml

----
-name: load_balancing_for_update_all_mirrors_worker
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/64526
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/334162
-milestone: '14.1'
-type: development
-group: group::source code
-default_enabled: true

ee/app/workers/update_all_mirrors_worker.rb

@@ -6,7 +6,7 @@ class UpdateAllMirrorsWorker # rubocop:disable Scalability/IdempotentWorker
   include CronjobQueue
 
   feature_category :source_code_management
-  data_consistency :sticky, feature_flag: :load_balancing_for_update_all_mirrors_worker
+  data_consistency :sticky
 
   LEASE_TIMEOUT = 5.minutes
   SCHEDULE_WAIT_TIMEOUT = 4.minutes

ee/spec/workers/update_all_mirrors_worker_spec.rb

@@ -11,10 +11,7 @@ RSpec.describe UpdateAllMirrorsWorker do
     stub_exclusive_lease
   end
 
-  it_behaves_like 'worker with data consistency',
-                  described_class,
-                  feature_flag: :load_balancing_for_update_all_mirrors_worker,
-                  data_consistency: :sticky
+  it_behaves_like 'worker with data consistency', described_class, data_consistency: :sticky
 
   describe '#perform' do
     it 'does nothing if the database is read-only' do