Remove unused feature flag mr_default_description_from_repo

MR: !91786

ee/spec/services/merge_requests/build_service_spec.rb

@@ -73,10 +73,6 @@
         let(:files) { { '.gitlab/merge_request_templates/Default.md' => 'Default template contents' } }
         let(:project) { create(:project, :repository, :custom_repo, merge_requests_template: template, files: files ) }
 
-        before do
-          stub_feature_flags(mr_default_description_from_repo: true)
-        end
-
         it 'prefers the project default template' do
           expect(merge_request.description).to eq(template)
         end