Don't use threadsafe variant in signal_event

[agners]

All calls to signal_event are made in the asyncio event loop thread. Therefore, we don't need to use the threadsafe variant of call_soon() anymore.

This was anyways a bit brittle as asyncio.create_task() is not threadsafe, so calling signal_event from a different thread was not really safe.

[agners]

Afaik, in practice this won't really make a difference as it is ok to call call_soon_threadsafe() from the event loop thread. But we'll get a runtime exception in the future in case we (accidentially) call signal_event() from another thread again.