class DeleteForm

1. 8.x drupal/core/modules/forum/lib/Drupal/forum/Form/DeleteForm.php \Drupal\forum\Form\DeleteForm
2. 8.x drupal/core/modules/contact/lib/Drupal/contact/Form/DeleteForm.php \Drupal\contact\Form\DeleteForm
3. 8.x drupal/core/modules/views_ui/lib/Drupal/views_ui/Form/DeleteForm.php \Drupal\views_ui\Form\DeleteForm
4. 8.x drupal/core/modules/path/lib/Drupal/path/Form/DeleteForm.php \Drupal\path\Form\DeleteForm
5. 8.x drupal/core/modules/action/lib/Drupal/action/Form/DeleteForm.php \Drupal\action\Form\DeleteForm

contact.routing.yml in drupal/core/modules/contact/contact.routing.yml

drupal/core/modules/contact/contact.routing.yml

class DeleteForm extends ConfirmFormBase {

  /**
   * The contact category being deleted.
   *
   * @var \Drupal\contact\Plugin\Core\Entity\Category
   */
  protected $contactCategory;

  /**
   * Implements \Drupal\Core\Form\FormInterface::getFormID().
   */
  public function getFormID() {
    return 'contact_category_delete_form';
  }

  /**
   * Implements \Drupal\Core\Form\ConfirmFormBase::getQuestion().
   */
  protected function getQuestion() {
    return t('Are you sure you want to delete %name?', array(
      '%name' => $this->contactCategory
        ->label(),
    ));
  }

  /**
   * Implements \Drupal\Core\Form\ConfirmFormBase::getCancelPath().
   */
  protected function getCancelPath() {
    return 'admin/structure/contact';
  }

  /**
   * Overrides \Drupal\Core\Form\ConfirmFormBase::getConfirmText().
   */
  protected function getConfirmText() {
    return t('Delete');
  }

  /**
   * Overrides \Drupal\Core\Form\ConfirmFormBase::buildForm().
   */
  public function buildForm(array $form, array &$form_state, Category $contact_category = NULL) {
    $this->contactCategory = $contact_category;
    return parent::buildForm($form, $form_state);
  }

  /**
   * Implements \Drupal\Core\Form\FormInterface::submitForm().
   */
  public function submitForm(array &$form, array &$form_state) {
    $this->contactCategory
      ->delete();
    drupal_set_message(t('Category %label has been deleted.', array(
      '%label' => $this->contactCategory
        ->label(),
    )));
    watchdog('contact', 'Category %label has been deleted.', array(
      '%label' => $this->contactCategory
        ->label(),
    ), WATCHDOG_NOTICE);
    $form_state['redirect'] = 'admin/structure/contact';
  }

}