Remove tmp_idx_vuln_reads_where_dismissal_reason_null index

Why are we doing this work

After Backfill `dismissal_reason` column for `vulnera... (!123518 - merged) is done, tmp_idx_vuln_reads_where_dismissal_reason_null index is no longer needed.

Implementation plan

1. Write a post-deployment migration to drop tmp_idx_vuln_reads_where_dismissal_reason_null