Remove 2nd stub expectation of #last_diff_file

It looks to be a stub/mock rather than strictly an expectation of the
system, so dropping this to only a single invocation expected, as we've
removed one of the two places #last_diff_file would be invoked.

spec/services/suggestions/create_service_spec.rb

@@ -96,7 +96,7 @@ describe Suggestions::CreateService do
 
       it 'creates no suggestion when diff file is not found' do
         expect_next_instance_of(DiffNote) do |diff_note|
-          expect(diff_note).to receive(:latest_diff_file).twice { nil }
+          expect(diff_note).to receive(:latest_diff_file).once { nil }
         end
 
         expect { subject.execute }.not_to change(Suggestion, :count)