Remove the project_cleanup feature flag

What does this MR do?

Removes the project cleanup feature flag. This hides the functionality implemented in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23189 https://gitlab.com/gitlab-org/gitlab-ce/issues/19376 , so should be merged prior to the 19th.

We also fix one bug in the project cleanup RPC client that was leading to an infinite loop around io.read and 100% CPU usage if the BFG object map file had been uploaded to object storage.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• 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

Closes #55230 (closed)