void unregisterRemoveListener(GlobalKey key, GlobalKeyRemoveListener listener)

Stop calling listener whenever a widget with the given global key is removed from the tree.

Listeners can be added with registerRemoveListener.

Source 

static void unregisterRemoveListener(GlobalKey key, GlobalKeyRemoveListener listener) {
  assert(key != null);
  assert(_removeListeners.containsKey(key));
  assert(_removeListeners[key].contains(listener));
  bool removed = _removeListeners[key].remove(listener);
  if (_removeListeners[key].isEmpty)
    _removeListeners.remove(key);
  assert(removed);
}