Geo - Improve Geo::RepositoriesCleanUpWorker

What does this MR do?

Check if project repository exists on the secondary node before scheduling removal.

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

No.

Why was this MR needed?

There is a possibility that a project does not have repository or wiki on a secondary node and we'll just end up enqueuing them on Sidekiq.

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?

Closes #3132 (closed)

/cc @stanhu @brodock @to1ne