Drop matching_merge_request_db_sync feature flag

This was shipped and enabled by default in GitLab 13.5, so we can drop
this feature flag. This merge request only affects sites that use the EE
database load balancing.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/254488

ee/changelogs/unreleased/sh-remove-matching-mr-db-sync-ff.yml

+---
+title: Drop matching_merge_request_db_sync feature flag
+merge_request: 49644
+author:
+type: changed

ee/config/feature_flags/development/matching_merge_request_db_sync.yml

----
-name: matching_merge_request_db_sync
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/42435
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/254488
-milestone: '13.5'
-type: development
-group: group::create
-default_enabled: true

ee/lib/ee/gitlab/checks/matching_merge_request.rb

@@ -11,7 +11,6 @@ module EE
 
         override :match?
         def match?
-          return super unless ::Feature.enabled?(:matching_merge_request_db_sync, default_enabled: true)
           return super unless ::Gitlab::Database::LoadBalancing.enable?
 
           # When a user merges a merge request, the following sequence happens:

ee/spec/lib/gitlab/checks/matching_merge_request_spec.rb

@@ -40,14 +40,15 @@ RSpec.describe Gitlab::Checks::MatchingMergeRequest do
 
     context 'with load balancing enabled', :request_store, :redis do
       let(:session) { ::Gitlab::Database::LoadBalancing::Session.current }
+      let(:all_caught_up) { true }
 
-      context 'on secondary that has caught up to primary' do
-        before do
-          allow(::Gitlab::Database::LoadBalancing::Sticking).to receive(:all_caught_up?).and_return(true)
-          expect(::Gitlab::Database::LoadBalancing).to receive(:enable?).and_return(true)
-          expect(::Gitlab::Database::LoadBalancing::Sticking).to receive(:unstick_or_continue_sticking).and_call_original
-        end
+      before do
+        expect(::Gitlab::Database::LoadBalancing).to receive(:enable?).and_return(true)
+        expect(::Gitlab::Database::LoadBalancing::Sticking).to receive(:unstick_or_continue_sticking).and_call_original
+        allow(::Gitlab::Database::LoadBalancing::Sticking).to receive(:all_caught_up?).and_return(all_caught_up)
+      end
 
+      context 'on secondary that has caught up to primary' do
         it 'continues to use the secondary' do
           expect(session.use_primary?).to be false
           expect(subject.match?).to be true
@@ -61,44 +62,17 @@ RSpec.describe Gitlab::Checks::MatchingMergeRequest do
       end
 
       context 'on secondary behind primary' do
-        before do
-          allow(::Gitlab::Database::LoadBalancing::Sticking).to receive(:all_caught_up?).and_return(false)
-        end
-
-        context 'with feature flag disabled' do
-          before do
-            stub_feature_flags(matching_merge_request_db_sync: false)
-            expect(::Gitlab::Database::LoadBalancing).not_to receive(:enable?)
-            expect(::Gitlab::Database::LoadBalancing::Sticking).not_to receive(:unstick_or_continue_sticking)
-          end
+        let(:all_caught_up) { false }
 
-          it 'does not check load balancing state' do
-            expect(subject.match?).to be true
-          end
-
-          it 'increments no counters' do
-            expect { subject.match? }
-              .to change { total_counter.get }.by(0)
-              .and change { stale_counter.get }.by(0)
-          end
+        it 'sticks to the primary' do
+          expect(subject.match?).to be true
+          expect(session.use_primary?).to be true
         end
 
-        context 'load balancing enabled' do
-          before do
-            expect(::Gitlab::Database::LoadBalancing).to receive(:enable?).and_return(true)
-            expect(::Gitlab::Database::LoadBalancing::Sticking).to receive(:unstick_or_continue_sticking).and_call_original
-          end
-
-          it 'sticks to the primary' do
-            expect(subject.match?).to be true
-            expect(session.use_primary?).to be true
-          end
-
-          it 'increments both total and stale counters' do
-            expect { subject.match? }
-              .to change { total_counter.get }.by(1)
-              .and change { stale_counter.get }.by(1)
-          end
+        it 'increments both total and stale counters' do
+          expect { subject.match? }
+            .to change { total_counter.get }.by(1)
+            .and change { stale_counter.get }.by(1)
         end
       end
     end