Remove :hset_redis_diff_caching feature flag

This removes the code instances of :hset_redis_diff_caching, as it was
fully enabled in 12.7

lib/gitlab/diff/file_collection/merge_request_diff_base.rb

@@ -47,11 +47,7 @@ module Gitlab
         private
 
         def cache
-          @cache ||= if Feature.enabled?(:hset_redis_diff_caching, project, default_enabled: true)
-                       Gitlab::Diff::HighlightCache.new(self)
-                     else
-                       Gitlab::Diff::DeprecatedHighlightCache.new(self)
-                     end
+          @cache ||= Gitlab::Diff::HighlightCache.new(self)
         end
       end
     end

spec/lib/gitlab/diff/file_collection/merge_request_diff_spec.rb

@@ -38,21 +38,6 @@ describe Gitlab::Diff::FileCollection::MergeRequestDiff do
     let(:diffable) { merge_request.merge_request_diff }
   end
 
-  context 'using Gitlab::Diff::DeprecatedHighlightCache' do
-    before do
-      stub_feature_flags(hset_redis_diff_caching: false)
-    end
-
-    it 'uses a different cache key if diff line keys change' do
-      mr_diff = described_class.new(merge_request.merge_request_diff, diff_options: nil)
-      key = mr_diff.cache_key
-
-      stub_const('Gitlab::Diff::Line::SERIALIZE_KEYS', [:foo])
-
-      expect(mr_diff.cache_key).not_to eq(key)
-    end
-  end
-
   it_behaves_like 'diff statistics' do
     let(:collection_default_args) do
       { diff_options: {} }

spec/services/merge_requests/reload_diffs_service_spec.rb

@@ -33,34 +33,13 @@ describe MergeRequests::ReloadDiffsService, :use_clean_rails_memory_store_cachin
     end
 
     context 'cache clearing' do
-      context 'using Gitlab::Diff::DeprecatedHighlightCache' do
-        before do
-          stub_feature_flags(hset_redis_diff_caching: false)
-        end
+      it 'clears the cache for older diffs on the merge request' do
+        old_diff = merge_request.merge_request_diff
+        old_cache_key = old_diff.diffs_collection.cache_key
 
-        it 'clears the cache for older diffs on the merge request' do
-          old_diff = merge_request.merge_request_diff
-          old_cache_key = old_diff.diffs_collection.cache_key
+        expect_any_instance_of(Redis).to receive(:del).with(old_cache_key).and_call_original
 
-          expect(Rails.cache).to receive(:delete).with(old_cache_key).and_call_original
-
-          subject.execute
-        end
-      end
-
-      context 'using Gitlab::Diff::HighlightCache' do
-        before do
-          stub_feature_flags(hset_redis_diff_caching: true)
-        end
-
-        it 'clears the cache for older diffs on the merge request' do
-          old_diff = merge_request.merge_request_diff
-          old_cache_key = old_diff.diffs_collection.cache_key
-
-          expect_any_instance_of(Redis).to receive(:del).with(old_cache_key).and_call_original
-
-          subject.execute
-        end
+        subject.execute
       end
 
       it 'avoids N+1 queries', :request_store do

spec/services/notes/create_service_spec.rb

@@ -87,28 +87,10 @@ describe Notes::CreateService do
           .to receive(:unfolded_diff?) { true }
       end
 
-      context 'using Gitlab::Diff::DeprecatedHighlightCache' do
-        before do
-          stub_feature_flags(hset_redis_diff_caching: false)
-        end
-
-        it 'clears noteable diff cache when it was unfolded for the note position' do
-          expect_any_instance_of(Gitlab::Diff::DeprecatedHighlightCache).to receive(:clear)
-
-          described_class.new(project_with_repo, user, new_opts).execute
-        end
-      end
+      it 'clears noteable diff cache when it was unfolded for the note position' do
+        expect_any_instance_of(Gitlab::Diff::HighlightCache).to receive(:clear)
 
-      context 'using Gitlab::Diff::HighlightCache' do
-        before do
-          stub_feature_flags(hset_redis_diff_caching: true)
-        end
-
-        it 'clears noteable diff cache when it was unfolded for the note position' do
-          expect_any_instance_of(Gitlab::Diff::HighlightCache).to receive(:clear)
-
-          described_class.new(project_with_repo, user, new_opts).execute
-        end
+        described_class.new(project_with_repo, user, new_opts).execute
       end
 
       it 'does not clear cache when note is not the first of the discussion' do