gradient_server-1.0.x-dev/modules/contact_apply_for_role/contact_apply_for_role.module

<?php

use Drupal\Core\Form\FormStateInterface;

function contact_apply_for_role_form_contact_message_apply_for_role_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  unset ($form['role']['widget']['#options']['_none']);
  unset ($form['role']['widget']['#options']['administrator']);
  unset ($form['role']['widget']['#options']['anonymous']);
  unset ($form['role']['widget']['#options']['authenticated']);
}