Delete MR files separately on project deletion
What does this MR do and why?
Deleting merge request files is causing timeouts sometimes. In order to fix this issue, let's remove the files in batches.
- Inspired by: !75963 (merged)
- Related issue: #352511 (closed)
- Sentry issue: https://sentry.gitlab.net/gitlab/gitlabcom/issues/3308882