Overrides Drupal\Core\Entity\EntityFormController::actions().
Overrides EntityFormController::actions
protected function actions(array $form, array &$form_state) {
$actions = parent::actions($form, $form_state);
$menu = $this->entity;
$system_menus = menu_list_system_menus();
$actions['delete']['#access'] = !$menu
->isNew() && !isset($system_menus[$menu
->id()]);
return $actions;
}