remove mention of nonexistent `_margv` in macro

closes  #3476

IPython/core/macro.py

@@ -19,8 +19,6 b' class Macro(object):'
 
     Macro is just a callable that executes a string of IPython
     input when called.
-    
-    Args to macro are available in _margv list if you need them.
     """
 
     def __init__(self,code):