Remove BackupRepos RPC
The streaming RPC BackupRepos is being replaced with BackupRepository as it was determined that streaming multiple repositories through praefect would be too difficult. Instead we'll use a simple single repository RPC.
BackupRepos is part of the server-side backups work that is not functional yet. So it is safe to remove the RPC.
This reverts commit 11825b3a, reversing changes made to 760051f1.
Edited by James Fargher