Delete Git repository references on MR delete
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=474401) </details> <!--IssueSummary end--> ### Background In https://gitlab.com/gitlab-org/gitlab/-/merge_requests/71493, it was explored that we can delete references created when a MR is created when that specific MR is deleted. This helps reduce the references in repository. ### Proposed Solution Continue https://gitlab.com/gitlab-org/gitlab/-/merge_requests/71493 to apply suggestion made in https://gitlab.com/gitlab-org/gitlab/-/merge_requests/71493/diffs#note_1496380658 or create a new MR.
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