Cleanup stale +deleted repo paths on project removal (adjusts project removal bug)

What does this MR do?

https://gitlab.com/gitlab-org/gitlab-ce/issues/46146#note_84727526 discussions show that:

1. When removing projects, we can end-up leaving the +deleted repo path dirty and not successfully removing the non-deleted namespace (mv process is not atomic and can be killed without fully moving the path).
2. In order to solve that, we're adding a clean-up phase on ensure which will schedule possible staled +deleted path deletion. Note that we don't check the current state (if there is or not a repo) in order to schedule the deletion. That's intentional in order to leverage Gitlab::GitalyClient::NamespaceService#remove idempotency and ensure consistency.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/46146

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer