Fix repo size limit stub in MergeToRefService spec

The project size limit check was changed a while back from being a
method on the project model (`above_size_limit?`) to using a service
(`Gitlab::RepositorySizeChecker`).

ee/spec/services/merge_requests/merge_to_ref_service_spec.rb

@@ -13,16 +13,12 @@ RSpec.describe MergeRequests::MergeToRefService do
   end
 
   describe '#execute' do
-    context 'project has exceeded size limit' do
-      before do
-        allow(project).to receive(:above_size_limit?).and_return(true)
-      end
+    it 'does not check the repository size limit' do
+      expect(project.repository_size_checker).not_to receive(:above_size_limit?)
 
-      it 'bypasses the repository limit check' do
-        result = service.execute(merge_request)
+      result = service.execute(merge_request)
 
-        expect(result[:status]).to eq(:success)
-      end
+      expect(result[:status]).to eq(:success)
     end
 
     context 'when no commit message is explicitly given and push rule is set' do