Remove unused index for container scanning reports

What does this MR do?

Part of #239311 (closed)

== 20201002094617 RemoveContainerScanningReportTypeIndex: migrating ===========
-- transaction_open?()
   -> 0.0000s
-- indexes(:vulnerability_occurrences)
   -> 0.0103s
-- execute("SET statement_timeout TO 0")
   -> 0.0003s
-- remove_index(:vulnerability_occurrences, {:algorithm=>:concurrently, :name=>"idx_container_scanning_findings"})
   -> 0.0069s
-- execute("RESET ALL")
   -> 0.0003s
== 20201002094617 RemoveContainerScanningReportTypeIndex: migrated (0.0187s) ==

Conformity

• Changelog entry
• Code review guidelines
• Style guides
• Database guides