Delete non-latest merge request diff files upon MRs merge

What does this MR do?

Deletes non-latest merge request diff files upon MRs merge.

This MR doesn't cover the background migration which will cleanup diff files from old MRs (ongoing on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20013).

Reviewer should double-check

MergeRequest#merge_request_diff is safe to use in this context? I believe it'll always return a record, since we create a new merge request diff before deleting the non-latest.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/37639