code: protected static <E extends Event> void unsubscribeSystemEvent(Class<E> eventClass, final Listener<E> listener) {
          SystemEventBus.unsubscribe(eventClass, listener);
      }
docs: <p>unsubscribeSystemEvent.</p>
  
  @param eventClass a {@link java.lang.Class} object.
  @param listener   a {@link ameba.event.Listener} object.
  @param <E> a E object.