code: func (s *IdentityService) DeleteSignupToken(token string) error {
  	if token == "" {
  		return trace.BadParameter("missing parameter token")
  	}
  	err := s.Delete(context.TODO(), backend.Key(userTokensPrefix, token))
  	return trace.Wrap(err)
  }
docs: // DeleteSignupToken deletes signup token from the storage