Rollup merge of #141969 - fmease:triagebot-rm-assign-users-on-vacation, r=Kobzol

Triagebot: Remove `assign.users_on_vacation`

It's been superseded by triagebot's [review queue tracking](https://forge.rust-lang.org/triagebot/review-queue-tracking.html), more specifically *rotation mode*.

r? Kobzol or triagebot

Author: matthiaskrgr