Cleanup feature flag iid_always_track

Closes https://gitlab.com/gitlab-org/gitlab/issues/30515

app/models/concerns/atomic_internal_id.rb

@@ -57,8 +57,7 @@ module AtomicInternalId
       end
 
       define_method("track_#{scope}_#{column}!") do
-        iid_always_track = Feature.enabled?(:iid_always_track, default_enabled: true)
-        return unless @internal_id_needs_tracking || iid_always_track
+        return unless @internal_id_needs_tracking
 
         scope_value = internal_id_read_scope(scope)
         return unless scope_value

changelogs/unreleased/ab-iid-unnecessary-locks.yml

+---
+title: Avoid unnecessary locks on internal_ids
+merge_request: 18328
+author:
+type: performance

spec/models/concerns/atomic_internal_id_spec.rb

@@ -22,11 +22,6 @@ describe AtomicInternalId do
     end
 
     context 'when value is set by ensure_project_iid!' do
-      context 'with iid_always_track false' do
-        before do
-          stub_feature_flags(iid_always_track: false)
-        end
-
       it 'does not track the value' do
         expect(InternalId).not_to receive(:track_greatest)
 
@@ -45,20 +40,6 @@ describe AtomicInternalId do
         subject
       end
     end
-
-      context 'with iid_always_track enabled' do
-        before do
-          stub_feature_flags(iid_always_track: true)
-        end
-
-        it 'does not track the value' do
-          expect(InternalId).to receive(:track_greatest)
-
-          milestone.ensure_project_iid!
-          subject
-        end
-      end
-    end
   end
 
   describe '#ensure_project_iid!' do

spec/support/shared_examples/models/atomic_internal_id_shared_examples.rb

@@ -47,10 +47,6 @@ shared_examples_for 'AtomicInternalId' do |validate_presence: true|
     end
 
     describe 'internal id generation' do
-      before do
-        stub_feature_flags(iid_always_track: false)
-      end
-
       subject { instance.save! }
 
       it 'calls InternalId.generate_next and sets internal id attribute' do