ConfirmFormBase::getCancelPath() has been replaced with getCancelRoute()

\Drupal\Core\Form\ConfirmFormBase is the Drupal 8 equivalent of confirm_form(). See the change notice for that conversion here: confirm_form() deprecated in favor of \Drupal\Core\Form\ConfirmFormBase

Instead of specifying the path to return to when the "Cancel" button is clicked, you now specify the route name, and optionally, any route parameters or options.

use Drupal\Core\Url;

  public function getCancelUrl() {
    return new Url('system_admin', array(), array(
        'query' => array(
          'destination' => 'admin/config',
        ),
      ),
    );
  }