Cleanup template labels after backfill is complete

In app/controllers/admin/labels_controller.rb we have a fallback for the update action to set organization_id to the current organization if not already set in the DB. This will no longer be necessary after the backfill is run. So we can remove this after the next required stop (18.5), so scheduling to 18.6

We also need to remove the template_labels_scoped_by_org in 18.6 as only then we can be sure the backfill has been run in a post deployment migration for self hosted instances