Clean up background migration BackfillIntegrationsEnableSslVerification

What does this MR do and why?

Finalize the migration and remove the temporary index.

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 #360336 (closed)