Hide fieldset relations in taxonomy term forms for flat hierarchy

mymodule.module

<?php 

/**
 * Implements hook_form_FORM_ID_alter().
 */
function mymodule_form_taxonomy_my_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  // Hide fieldset relations for flat hierarchy taxonomies.
  $form['relations']['#required'] = FALSE;
  $form['relations']['#access'] = FALSE;
}