Allow restoring into a non-existent repo on praefect

Fixes #3775 (closed)

Praefect expects to be able to route the call to RemoveRepository to the correct nodes, but fails when it cannot find the repository record.

time="2021-09-08T19:23:38Z" level=error msg="restore failed" error="manager: remove repository: rpc error: code = Unknown desc = mutator call: route repository mutator: get primary: repository \"default\"/\"@groups/d4/73/d4735e3a265e16eee03f59718b9b5d03019c07d8b6c51f90da3a666eec13ab35.wiki.git\" not found" gl_project_path=gitlab-instance-74bf01f9.wiki relative_path=@groups/d4/73/d4735e3a265e16eee03f59718b9b5d03019c07d8b6c51f90da3a666eec13ab35.wiki.git storage_name=default