Remove temporary index tmp_idx_vulnerability_occurrences

What does this MR do and why?

This MR removes a temporary index introduced in !112556 (merged)

How to set up and validate locally

Before the migration:

1. Run bin/rails dbconsole
2. \d vulnerability_occurrences
3. Check that index is present: "tmp_idx_vulnerability_occurrences_on_id_where_report_type_7_99" btree (id) WHERE report_type = ANY (ARRAY[7, 99])

Migrate and check for removed index:

1. Run bin/rails db:migrate
2. Run bin/rails dbconsole
3. \d vulnerability_occurrences
4. Check that index tmp_idx_vulnerability_occurrences_on_id_where_report_type_7_99 is not listed anymore

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #393052 (closed)