Draft: Resolve "Remove temporary index added by "Add temp index for notes without discussion_id""

What does this MR do and why?

The temporary index is not needed anymore as we've shifted toward a different solution which does not use it.

It would only be needed in a manual query of a Change Request. But we've decided to not go forward with the Change Request after all.

Database Migration

TBD

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 #357581 (closed)