Remove deprecated child watcher (#7549)

It looks like this was added originally to fix some tests (maybe no
watcher was set by default in asyncio at the time...?). So, as long as
the tests are passing, it should be safe to remove this code.

aiohttp/test_utils.py

@@ -465,19 +465,7 @@ def setup_test_loop(
     once they are done with the loop.
     """
     loop = loop_factory()
-    try:
-        module = loop.__class__.__module__
-        skip_watcher = "uvloop" in module
-    except AttributeError:  # pragma: no cover
-        # Just in case
-        skip_watcher = True
     asyncio.set_event_loop(loop)
-    if sys.platform != "win32" and not skip_watcher:
-        policy = asyncio.get_event_loop_policy()
-        watcher = asyncio.ThreadedChildWatcher()
-        watcher.attach_loop(loop)
-        with contextlib.suppress(NotImplementedError):
-            policy.set_child_watcher(watcher)
     return loop