Tidy up specs that dont need a diff to be created

Now that we have the variable 'skip_ensure_merge_request_diff` we can remove the special class we were using to stop the creation of a diff.

spec/factories/merge_requests.rb

@@ -314,9 +314,9 @@
     end
 
     trait :skip_diff_creation do
-      # rubocop: disable Lint/UselessAssignment
-      skip_ensure_merge_request_diff = true
-      # rubocop: enable Lint/UselessAssignment
+      before(:create) do |merge_request, _|
+        merge_request.skip_ensure_merge_request_diff = true
+      end
     end
 
     after(:build) do |merge_request|