Drop tmp_index_vulnerabilities_on_id_finding_id_empty index

Why are we doing this work?

After Backfill `finding_id` foreign key on `vulnerabi... (#418971 - closed) is complete we need to remove tmp_index_vulnerabilities_on_id_finding_id_empty supporting index

Implementation plan

1. Create a post-deployment migration to drop tmp_index_vulnerabilities_on_id_finding_id_empty index