chore: drop Python2-specific alias

Author: tseaver

google/api_core/grpc_helpers.py

@@ -92,7 +92,7 @@ def __iter__(self):
         """This iterator is also an iterable that returns itself."""
         return self
 
-    def next(self):
+    def __next__(self):
         """Get the next response from the stream.
 
         Returns:
@@ -108,9 +108,6 @@ def next(self):
             # If the stream has already returned data, we cannot recover here.
             raise exceptions.from_grpc_error(exc) from exc
 
-    # Alias needed for Python 2/3 support.
-    __next__ = next
-
     # grpc.Call & grpc.RpcContext interface
 
     def add_callback(self, callback):