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

The following discussion from !84038 (merged) should be addressed:

• @Alexand started a discussion:

  Could we create already the issue to track the removal of the temporary index, and add a comment as suggested in our docs?

  The docs actually exemplify on a sync migration, but since we're effectively creating the index with this async one, I wondered if it would be worth to create it already and add the comment. WDYT?