* Fix Backport Assistant failure PR commenting
For general comments on a PR, it looks like you have to use the `/issue`
endpoint rather than `/pulls`, which requires commit/other
review-specific target details.
This matches the endpoint used in `backport-reminder.yml`.
* Remove Backport Reminder workflow
This is noisy (even when adding multiple labels, individual comments per
label are generated), and likely no longer needed: we haven't had this
work in a long time due to an expired GH token, and we now have better
automation for backport PR assignment.
1c7fcdf188