[3.12] remove redundant call to attach_loop in watcher (GH-110847) (#110867)

(cherry picked from commit 5965891)

Co-authored-by: Thomas Grainger <[email protected]>

Author: miss-islington

Lib/asyncio/unix_events.py

@@ -1461,8 +1461,6 @@ def _init_watcher(self):
                     self._watcher = PidfdChildWatcher()
                 else:
                     self._watcher = ThreadedChildWatcher()
-                if threading.current_thread() is threading.main_thread():
-                    self._watcher.attach_loop(self._local._loop)
 
     def set_event_loop(self, loop):
         """Set the event loop.