No need to null the loop reference during cleanup

This removes a small (but inconsequential) race on shutdown, where the
loop thread could access None when trying to start.

PYTHON-578

cassandra/io/libevreactor.py

@@ -135,7 +135,6 @@ class LibevLoop(object):
                 "Please call Cluster.shutdown() to avoid this.")
 
         log.debug("Event loop thread was joined")
-        self._loop = None
 
     def add_timer(self, timer):
         self._timers.add_timer(timer)