Don't use tp.definition.arguments in pretty_callable

This is a partial revert of d180456.

Resolves #11899

Author: fperrin

mypy/messages.py

@@ -1948,9 +1948,9 @@ def [T <: int] f(self, x: int, y: T) -> None
 
     # If we got a "special arg" (i.e: self, cls, etc...), prepend it to the arg list
     if isinstance(tp.definition, FuncDef) and tp.definition.name is not None:
-        definition_args = [arg.variable.name for arg in tp.definition.arguments]
+        definition_args = tp.definition.arg_names
         if definition_args and tp.arg_names != definition_args \
-                and len(definition_args) > 0 and definition_args[0]:
+                and len(definition_args) > 0:
             if s:
                 s = ', ' + s
             s = definition_args[0] + s