Some small refactorings to the names_dict/deep_ast_copy logic.

jedi/parser/tree.py

@@ -1193,10 +1193,6 @@ class CompFor(BaseNode):
             arr.append(name)
         return dct
 
-    @names_dict.setter
-    def names_dict(self, value):
-        pass
-
     def names_dicts(self, search_global):
         yield self.names_dict