unregister method
Removes route from the registry.
Implementation
void unregister(ModalRoute<dynamic> route) {
if (_registry.remove(route) == null) return;
_unlisten(route.animation);
_unlisten(route.secondaryAnimation);
final listener = _clockListeners.remove(route);
if (listener != null) route.animation?.removeStatusListener(listener);
if (identical(_clockRoute, route)) _clockRoute = null;
_scheduleNotify();
}