Remove python2-only __nonzero__ method from cluster.py (#2313)

Author: sobolevn

redis/cluster.py

@@ -1755,10 +1755,6 @@ def __len__(self):
         """ """
         return len(self.command_stack)
 
-    def __nonzero__(self):
-        "Pipeline instances should  always evaluate to True on Python 2.7"
-        return True
-
     def __bool__(self):
         "Pipeline instances should  always evaluate to True on Python 3+"
         return True