Draft: Remove tmp index from vulnerability_reads for bbm

What does this MR do and why?

Now that the migration implemented by Add migration to resolve vulnerabilities from r... (!157776 - merged) • Adam Cohen • 17.3 has completed, we can remove the tmp_index_vulnerability_reads_where_state_is_detected from the vulnerability_reads table, which was originally added by !161296 (diffs)+s.

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Related to #475161