Remove merge_reg_auto_sync_lock feature flag

Removed as part of feature flag cleanup work. First introduced in
12.2...

app/services/merge_requests/mergeability_check_service.rb

@@ -38,7 +38,6 @@ module MergeRequests
     # error otherwise.
     def execute(recheck: false, retry_lease: true)
       return service_error if service_error
-      return check_mergeability(recheck) unless merge_ref_auto_sync_lock_enabled?
 
       in_write_lock(retry_lease: retry_lease) do |retried|
         # When multiple calls are waiting for the same lock (retry_lease),
@@ -157,10 +156,6 @@ module MergeRequests
       result[:status] == :success
     end
 
-    def merge_ref_auto_sync_lock_enabled?
-      Feature.enabled?(:merge_ref_auto_sync_lock, project, default_enabled: true)
-    end
-
     def service_error
       strong_memoize(:service_error) do
         if !merge_request

config/feature_flags/development/merge_ref_auto_sync_lock.yml

----
-name: merge_ref_auto_sync_lock
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/31082
-rollout_issue_url: 
-milestone: '12.2'
-type: development
-group: group::code review
-default_enabled: true

spec/services/merge_requests/mergeability_check_service_spec.rb

@@ -124,14 +124,6 @@ RSpec.describe MergeRequests::MergeabilityCheckService, :clean_gitlab_redis_shar
 
     it_behaves_like 'mergeable merge request'
 
-    context 'when lock is disabled' do
-      before do
-        stub_feature_flags(merge_ref_auto_sync_lock: false)
-      end
-
-      it_behaves_like 'mergeable merge request'
-    end
-
     context 'when concurrent calls' do
       it 'waits first lock and returns "cached" result in subsequent calls' do
         threads = execute_within_threads(amount: 3)