Extract MergeRequestDiffCommit deletions

What does this MR do and why?

Projects will have at least one merge_request_diff_commit for every commit contained in every MR, which deleting via project.destroy! and cascading deletes may exceed statement timeouts, causing failures.

This is a similar solution as !71342 (merged)

Related to #346166 (closed)

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #346166 (closed)