Remove unused wait events.
Commit
464824323e introduced the wait events which were neither used by
that commit nor by follow-up commits for that work.
Author: Masahiro Ikeda
Backpatch-through: 14, where it was introduced
Discussion: https://postgr.es/m/
ff077840-3ab2-04dd-bbe4-
4f5dfd2ad481@oss.nttdata.com