Remove indices with namespace_id on vulnerability_reads

We have moved to using traversal_ids column in vulnerability_reads to fetch group level report data in !157786 (merged).

We have the below indices which include namespace_id:

• "index_vulnerability_reads_on_namespace_type_severity_id" btree (namespace_id, report_type, severity, vulnerability_id)

• "index_vulnerability_reads_common_finder_query_w_namespace_id" btree (namespace_id, state, report_type, severity, vulnerability_id DESC, dismissal_reason)

• "index_vuln_reads_on_namespace_id_state_severity_and_vuln_id" btree (namespace_id, state, severity, vulnerability_id DESC)

as discussed in #465871 (comment 1979360288) these indices can be removed after verifying the FF rollout is stable on production.