asyncio: Remove atexit hook

This hook was added because of an only-in-CI issue, but we have since
improved our cleanup of the selector thread. As long as this passes
CI, I think we can remove the atexit hook.

Fixes #3291

tornado/platform/asyncio.py

@@ -23,7 +23,6 @@ the same event loop.
 """
 
 import asyncio
-import atexit
 import concurrent.futures
 import errno
 import functools
@@ -61,31 +60,6 @@ _FileDescriptorLike = Union[int, _HasFileno]
 _T = TypeVar("_T")
 
 
-# Collection of selector thread event loops to shut down on exit.
-_selector_loops: Set["SelectorThread"] = set()
-
-
-def _atexit_callback() -> None:
-    for loop in _selector_loops:
-        with loop._select_cond:
-            loop._closing_selector = True
-            loop._select_cond.notify()
-        try:
-            loop._waker_w.send(b"a")
-        except BlockingIOError:
-            pass
-        if loop._thread is not None:
-            # If we don't join our (daemon) thread here, we may get a deadlock
-            # during interpreter shutdown. I don't really understand why. This
-            # deadlock happens every time in CI (both travis and appveyor) but
-            # I've never been able to reproduce locally.
-            loop._thread.join()
-    _selector_loops.clear()
-
-
-atexit.register(_atexit_callback)
-
-
 class BaseAsyncIOLoop(IOLoop):
     def initialize(  # type: ignore
         self, asyncio_loop: asyncio.AbstractEventLoop, **kwargs: Any
@@ -480,7 +454,6 @@ class SelectorThread:
         self._waker_r, self._waker_w = socket.socketpair()
         self._waker_r.setblocking(False)
         self._waker_w.setblocking(False)
-        _selector_loops.add(self)
         self.add_reader(self._waker_r, self._consume_waker)
 
     def close(self) -> None:
@@ -492,7 +465,6 @@ class SelectorThread:
         self._wake_selector()
         if self._thread is not None:
             self._thread.join()
-        _selector_loops.discard(self)
         self.remove_reader(self._waker_r)
         self._waker_r.close()
         self._waker_w.close()