Remove temporary index for backfilling namespace routes

Related to !78393 (comment 836490832)

In !78393 (merged) was added a temporary index for batched background migration, we can remove this index when migration is done.

Details

This can be closed after have following 2 MRs merged:

• Finalize cleanup orphaned routes migration (!88629 - merged)
• Remove temporary index from routes table (!88637 - merged)
• validate not null constrain on routes#namesapce_id, see Add not null constraint on routes#namespace_id ... (!88635 - merged)