removeValue method
Removes a value from the node.
@param key the key of the value to be removed @return the removed node value or null if not found @throws StorageException if the storage backend encounters a problem
Implementation
@override
Future<NodeValue?> removeValue(String key) async {
await _init();
lastModified = DateTime.now().millisecondsSinceEpoch;
return _values.remove(key);
}