Clean up resource_link_events backfill related code and index

A BG migration backfilled resource_link_events table in %16.1 and the index/code used in the migration process should be cleaned up.

• Finalize the batched background migration !122014 (merged).

• Remove index tmp_index_for_backfilling_resource_link_events.

• [-] Remove the relation between resource_link_events and system_note_metadata.