code: public void deleteCustomAttribute(final Object userIdOrUsername, final CustomAttribute customAttribute) throws GitLabApiException {
          if (Objects.isNull(customAttribute)) {
              throw new IllegalArgumentException("customAttributes can't be null");
          }
  
          deleteCustomAttribute(userIdOrUsername, customAttribute.getKey());
      }
docs: Delete a custom attribute for the given user
  
  @param userIdOrUsername the user in the form of an Integer(ID), String(username), or User instance
  @param customAttribute to remove
  @throws GitLabApiException on failure while deleting customAttributes