Smarter system notes not working for merge requests

• https://gitlab.com/gitlab-org/gitlab-ce/issues/33166 not working for merge requests
• Reproduced in staging.
• It is working for issues and epics, but not for merge requests (inside staging).

Example of not working in merge requests: https://staging.gitlab.com/gitlab-org/gitlab-ce/merge_requests/12308#note_42968273