cross-region replication queries namespaces twice

While working on node pool replication, I noticed what looks like an inefficient query of namespaces where we `List` the namespaces in the remote region and then `Get` each namespace. But for namespaces, these RPCs return the same objects. This code may have been copied from ACL replication, but we should double-check if it can be removed.

Start with the namespace handling in cross-region replication, then compare it with the ACL replication code mentioned in the issue. Verify whether the List and Get RPCs return the same namespace objects. Done means removing the redundant lookup if confirmed and adding or updating coverage for namespace replication behavior.

Written by the indexing model from the issue text.