Use Gitaly's DeleteAllRepositories RPC during backup restore

What does this MR do?

This migrates a part of the gitlab:backup:restore task: the part where we delete all existing repositories before restoring repositories from the backup.

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

Why was this MR needed?

Because the old code did not use Gitaly.

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
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes gitaly#1194 (closed)