Apply refurb/ruff rule FURB111 (#11837)

Author: DimitriPapadopoulos

sphinx/util/cfamily.py

@@ -107,7 +107,7 @@ def _stringify(self, transform: StringifyTransform) -> str:
         raise NotImplementedError(repr(self))
 
     def __str__(self) -> str:
-        return self._stringify(lambda ast: str(ast))
+        return self._stringify(str)
 
     def get_display_string(self) -> str:
         return self._stringify(lambda ast: ast.get_display_string())