Outdated merge request discussion threads can't be fully deleted

Summary

Deleting a discussion on a merge requests results in the text of the comment being deleted, but the diff still appears in the merge request.

Steps to reproduce

1. Create a new merge request
2. Comment on a line in the diff
   1. Click on the Changes tab
   2. Click the Add a comment to this line icon button
   3. Type in some text and click Add comment now
3. Push another commit to the branch that changes the line that was commented on in the step above
4. Refresh the page
5. In the Overview tab, delete the comment using More actions > Delete comment

Example Project

See comments in !26011 (merged)

What is the current bug behavior?

After deleting the comment, an empty diff still appears in the merge request notes:

What is the expected correct behavior?

Nothing is rendered in the merge request notes.

Output of checks

This bug happens on GitLab.com.