[portage] config: remove non-existent method

Author: Tatsh

stubs/portage/portage/package/ebuild/config.pyi

@@ -21,4 +21,3 @@ class config:
     def __delitem__(self, key: str) -> None: ...
     def __iter__(self) -> str: ...
     def get(self, k: str, x: Any = ...) -> Any: ...
-    def __len__(self) -> int: ...