removeTokens method Null safety
- AuthServer server,
- int resourceOwnerID
override
    This method must delete all AuthToken and AuthCodes for a ResourceOwner.
server is the requesting AuthServer. resourceOwnerID is the ResourceOwner.id.
Implementation
@override
Future removeTokens(AuthServer server, int resourceOwnerID) {
  final tokenQuery = Query<ManagedAuthToken>(context!)
    ..where((o) => o.resourceOwner).identifiedBy(resourceOwnerID);
  return tokenQuery.delete();
}