src/vs/workbench/contrib/audioCues/browser/observable.ts/BaseObservable/unsubscribe
class BaseObservable:	public unsubscribe(observer: IObserver): void {
		const deleted = this.observers.delete(observer);
		if (deleted && this.observers.size === 0) {
			this.onLastObserverUnsubscribed();
		}
	}