Update the backfill migration as no-op and remove global_search_*_tab feature flags YAML files.

Background

Remove feature flags YAML files and update the backfill migration as no-op. This should be after the required stop because the backfill migration must be completed before removing the feature flags.

Proposal

1. Mark the backfill migration as a no-op
2. Remove all the global_search_*_tab feature flags YAML files