allow having duration in payload representer

It will only show up if the feature flag is enabled. As this will not yet be used in production it is ok to behave like that.

spec/lib/api/v3/work_packages/work_package_payload_representer_spec.rb

@@ -269,32 +269,6 @@ describe ::API::V3::WorkPackages::WorkPackagePayloadRepresenter do
           end
         end
       end
-
-      describe 'duration' do
-        before do
-          allow(OpenProject::FeatureDecisions)
-             .to receive(:work_packages_duration_field_active?)
-             .and_return(true)
-        end
-
-        it 'has no duration attribute' do
-          # TODO: Remove this, once the duration feature is complete
-          expect(subject).not_to have_json_path('duration')
-        end
-
-        context 'when duration feature flag disabled' do
-          before do
-            allow(OpenProject::FeatureDecisions)
-               .to receive(:work_packages_duration_field_active?)
-               .and_return(false)
-          end
-
-          it 'has no duration attribute' do
-            # TODO: Remove this, once the duration feature is complete
-            expect(subject).not_to have_json_path('duration')
-          end
-        end
-      end
     end
 
     describe '_links' do