[2.0.x] Removed Python 2 comment in ValidationError.

Backport of 7fb913c805 from master

django/core/exceptions.py

@@ -102,8 +102,6 @@ class ValidationError(Exception):
         list or dictionary can be an actual `list` or `dict` or an instance
         of ValidationError with its `error_list` or `error_dict` attribute set.
         """
-
-        # PY2 can't pickle naive exception: http://bugs.python.org/issue1692335.
         super().__init__(message, code, params)
 
         if isinstance(message, ValidationError):