LanguageScreen [nfc]: Inline handleFilterChange.

This wasn't doing any real work, and it's a bit less code to inline
it.

We're not losing the `useCallback`'s optimization here. The
`useCallback` doc has a "Note" box, after introducing the example of

```
const [state, setState] = useState(initialState);
```

The "Note" box says,

> React guarantees that `setState` function identity is stable and
> won’t change on re-renders.

[1] https://reactjs.org/docs/hooks-reference.html#usecallback

src/settings/LanguageScreen.js

@@ -28,10 +28,8 @@ export default function LanguageScreen(props: Props) {
     [dispatch],
   );
 
-  const handleFilterChange = useCallback((f: string) => setFilter(f), []);
-
   return (
-    <Screen search searchBarOnChange={handleFilterChange} scrollEnabled={false}>
+    <Screen search searchBarOnChange={setFilter} scrollEnabled={false}>
       <LanguagePicker value={locale} onValueChange={handleLocaleChange} filter={filter} />
     </Screen>
   );