Remove misleading comment

The comment originally referred to the __metaclass__ attribute which is gone now.

graphene/utils/subclass_with_meta.py

@@ -19,7 +19,6 @@ def __repr__(cls):
 class SubclassWithMeta(metaclass=SubclassWithMeta_Meta):
     """This class improves __init_subclass__ to receive automatically the options from meta"""
 
-    # We will only have the metaclass in Python 2
     def __init_subclass__(cls, **meta_options):
         """This method just terminates the super() chain"""
         _Meta = getattr(cls, "Meta", None)