Delete Git repository references on MR delete

What does this MR do and why?

Delete merge-request references when a merge request is deleted.

Note: As keep-around refs might exist, even if the merge request was not merged, commits won't be removed when running housekeeping

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A

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.