Implements hook_form_FORM_ID_alter() for taxonomy_term_form().
function forum_form_taxonomy_term_form_alter(&$form, &$form_state, $form_id) {
$vid = config('forum.settings')
->get('vocabulary');
if (isset($form['vid']['#value']) && $form['vid']['#value'] == $vid) {
// Hide multiple parents select from forum terms.
$form['relations']['parent']['#access'] = FALSE;
}
}