Delete invalid routes when namespace migration is complete

When both namespace and project routes have backfilled namespace_id (#346446 (closed)), there will some routes with namespace_id not set - these are invalid routes which were skipped (!78393 (comment 829818875)) during backfilling because their source didn't exist.

We can re-verify all of routes with nil namespace_id have invalid source and delete these.

IMPORTANT This issue depends on backfill migration finishing successfully, i.e. we need to run a finalize migration first #358956 (closed)