Redundant call to call_soon_threasafe when using PidfdChildWatcher

[graingert]

It would be better to move the responsibility to the child watchers for calling callbacks on the event loop thread. Currently the callback needs to defensively call call_soon_threadsafe which is redundant on the Pidfdchildwatcher

Eg

cpython/Lib/asyncio/unix_events.py

Lines 225 to 227 in 282edd7

	def _child_watcher_callback(self, pid, returncode, transp):
	# Skip one iteration for callbacks to be executed
	self.call_soon_threadsafe(self.call_soon, transp._process_exited, returncode)

Originally posted by @graingert in #94597 (comment)

[graingert]

This will probably need to wait for the deprecation of the child watcher system in #94597

[gvanrossum]

Yeah, I notied this too when @1st1 and I were reviewing the watcher code this morning. Not sure we need to keep an issue open to remind us of this though.

[kumaraditya303]

I doubt anything can be done for this until the entire child watcher system is removed and is harmless.

[kumaraditya303]

Will take care of it in 3.14 when removing child watchers, no need for an issue for it, I am already working on it.