Resolve "Deleting of a GitLab Pages project results in "`PagesWorker.perform_in` cannot be called inside a transaction""

What does this MR do?

Wraps the after_destroy :remove_pages call in a run_after_commit block to avoid a runtime error.

Also reworks the handling of remove_pages for namespaceless projects

Are there points in the code the reviewer needs to double check?

We don't put the run_after_commit block inside remove_pages because the method is also called directly from the PagesController

Why was this MR needed?

Fix a runtime exception

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #35072 (closed)