Cleans up mirror capacity if project is a scheduled mirror

What does this MR do?

If the project was a mirror and was already scheduled (meaning it has capacity reserved for him), we need to cleanup in the event of him being removed. Otherwise that capacity slot will never get released.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?