Remove FF merge_trains_api

This commit removes the FF

ee/lib/api/merge_trains.rb

@@ -5,7 +5,6 @@ module API
     include PaginationParams
 
     before do
-      service_unavailable! unless Feature.enabled?(:merge_trains_api, user_project, default_enabled: true)
       authorize_read_merge_trains!
     end
 

ee/spec/requests/api/merge_trains_spec.rb

@@ -84,18 +84,6 @@ describe API::MergeTrains do
           expect(response).to have_gitlab_http_status(:forbidden)
         end
       end
-
-      context 'when feature flag is disabled' do
-        before do
-          stub_feature_flags(merge_trains_api: false)
-        end
-
-        it 'forbids the request' do
-          subject
-
-          expect(response).to have_gitlab_http_status(:service_unavailable)
-        end
-      end
     end
   end
 end