Remove deprecated LoopRunner API

[fjetter]

We deprecated functionality around LoopRunner that allowed to pass closed/not-running loops to it, see #6680

It's been ~3 months now and we should remove the deprecated functionality. This also relates to tornado 6.2 and py3.11 compat, see #7134

Closes #6163

@wence- you started looking into this. Are you still interested?

cc @graingert

[wence-]

@wence- you started looking into this. Are you still interested?

Yes. I got some way in but then because quite confused about whose responsibility it should be to start the event loop. Removing the final make_current in Nanny._run wasn't sufficient to remove usage of deprecated asyncio functionality AFAICT.