src: fix cleanup hook removal for InspectorTimer

Fix this to account for the fact that `Stop()` may already have been
called from a cleanup hook when the `inspector::Agent` is deleted
along with the `Environment`, at which point cleanup hooks are no
longer available.

PR-URL: https://github.com/nodejs/node/pull/32523


Reviewed-By: Matheus Marchini <mat@mmarchini.me>
Reviewed-By: James M Snell <jasnell@gmail.com>