Remove redundant definition of ast.PyCF_ONLY_AST (#2803)

This gets imported from `_ast`, so the definition was
redundant. This causes problems with the new mypy semantic
analyzer, as it will flag the redefinition as an error.

stdlib/3/ast.pyi

@@ -27,5 +27,3 @@ def iter_child_nodes(node: AST) -> Iterator[AST]: ...
 def iter_fields(node: AST) -> Iterator[_typing.Tuple[str, Any]]: ...
 def literal_eval(node_or_string: Union[str, AST]) -> Any: ...
 def walk(node: AST) -> Iterator[AST]: ...
-
-PyCF_ONLY_AST = ...  # type: int