src: make implementing CancelPendingDelayedTasks for platform optional

Fold `CancelPendingDelayedTasks()` into `UnregisterIsolate()` and
make implementing it optional.

It makes sense for these two operations to happen at the same time,
so it is sufficient to provide a single operation instead of two
separate ones.

PR-URL: https://github.com/nodejs/node/pull/30034


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Shelley Vohr <codebytere@gmail.com>