Removed deprecated 'class' key from #theme links and #theme menu_tree heading arrays

For consistency throughout core, we are using the attributes key to apply ['attributes']['class'] to a heading on #theme links and #theme menu_tree instead of ['class'].

Before in D7

  $variables['secondary_menu'] = array(
      '#theme' =>'links__system_secondary_menu',
      '#links' => $variables['secondary_menu'],
      '#heading' => array(
        'text' => t('Secondary menu'),
        'class' => array('visually-hidden'),
        'attributes' => array('id' => 'links__system_secondary_menu'),
      ),
      '#set_active_class' => TRUE,
    );

After in D8

    $variables['secondary_menu'] = array(
      '#theme' =>'links__system_secondary_menu',
      '#links' => $variables['secondary_menu'],
      '#heading' => array(
        'text' => t('Secondary menu'),
        'attributes' => array(
          'id' => 'links__system_secondary_menu',
          'class' => array('visually-hidden'),
        ),
      ),
      '#set_active_class' => TRUE,
    );