Remove mrc_api_use_raw_diffs_from_gitaly flag

Changelog: removed

config/feature_flags/development/mrc_api_use_raw_diffs_from_gitaly.yml

----
-name: mrc_api_use_raw_diffs_from_gitaly
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/46190
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/225322
-type: development
-group: group::code review
-default_enabled: false
-milestone: '13.6'

lib/api/entities/merge_request_changes.rb

@@ -24,7 +24,7 @@ module API
       end
 
       def expose_raw_diffs?
-        options[:access_raw_diffs] || ::Feature.enabled?(:mrc_api_use_raw_diffs_from_gitaly, options[:project])
+        options[:access_raw_diffs]
       end
     end
   end

spec/requests/api/merge_requests_spec.rb

@@ -1605,11 +1605,7 @@ RSpec.describe API::MergeRequests do
       expect(json_response['overflow']).to be_falsy
     end
 
-    context 'when using DB-backed diffs via feature flag' do
-      before do
-        stub_feature_flags(mrc_api_use_raw_diffs_from_gitaly: false)
-      end
-
+    context 'when using DB-backed diffs' do
       it_behaves_like 'find an existing merge request'
 
       it 'accesses diffs via DB-backed diffs.diffs' do