[Charts] Remove/Hide `useDarkMode` hook from charts theme service

[nickofthyme]

In the charts plugin we exposed a useDarkMode hook to make it easy to fetch dark mode from charts instead of also having to import the theme.

kibana/src/platform/plugins/shared/charts/public/services/theme/theme.ts

Lines 35 to 47 in e269d04

	/** A React hook for consuming the dark mode value */
	public useDarkMode = (): boolean => {
	const [value, update] = useState(false);
	useEffect(() => {
	const s = this.darkModeEnabled$.subscribe((val) => {
	update(val.darkMode);
	});
	return () => s.unsubscribe();
	}, []);
	return value;
	};

In addition we also provided an observable for darkModeEnabled$.

kibana/src/platform/plugins/shared/charts/public/services/theme/theme.ts

Lines 26 to 33 in e269d04

	/** An observable boolean for dark mode of kibana */
	public get darkModeEnabled$(): Observable<CoreTheme> {
	if (!this.theme$) {
	throw new Error('ThemeService not initialized');
	}
	return this.theme$;
	}

With changes to correct usage of the theme across kibana in #220141 we deprecated these properties to prevent usage in favor of useKibanaIsDarkMode (later to be useIsDarkMode from eui).

These properties should eventually be hidden in the case of darkModeEnabled$ and removed in the case of useDarkMode.

[elasticmachine]

Pinging @elastic/kibana-visualizations (Team:Visualizations)