=per #15937 simplifies PR #16126 by simpler delegation from Processor

Hi there, thanks for your PR to akka!
I looked into it for a while and found a way to simplify the delegation in `Processor`.

We don't have to do anything with `aroundPreStart` and `preRestartDefault` is gone as well.

Tests pass for me, so if you'd like to please go ahead and merge this into your pull-request branch (which will update the PR in akka/akka), and we'll ask the team to re-review it. (You can squash this commit into yours).