Clean up background migration BackfillIntegrationsEnableSslVerification

Summary

Clean up the background migration from #349697 (closed) / !80302 (merged), as described on https://docs.gitlab.com/ee/development/database/background_migrations.html#cleaning-up

Improvements

• Finalize migration
• Drop temporary index added in !80302 (merged)

Availability & Testing

Suggestion: run package-and-qa