bpo-20526: Fix PyThreadState_Clear(): don't decref frame (GH-19120)

PyThreadState.frame is a borrowed reference, not a strong reference:
PyThreadState_Clear() must not call Py_CLEAR(tstate->frame).

Remove test_threading.test_warnings_at_exit(): we cannot warranty
that the Python thread state of daemon threads is cleared in a
reliable way during Python shutdown.