Cleanup members records with `member_namespace_id` null

When both namespace and project members have backfilled member_namespace_id, there will some members records with member_namespace_id not set - these are invalid members records which were skipped during backfilling because their source didn't exist, due to polymorphic source association.

We can re-verify all of members records with with NULL member_namespace_id have invalid source and delete these. see Cleanup orphaned routes (!88401 - merged) as an example

IMPORTANT This issue depends on backfill migration finishing successfully, i.e. we need to run a finalize migration first Finalise backfilling project namespace members ... (#365027 - closed)