ConfirmFormInterface::getFormName() serves no purpose in ConfirmFormBase

Problem/Motivation

ConfirmFormInterface::getFormName() is only called by ConfirmFormBase. The return value, a string, is used to build a hidden form element:

    $form[$this->getFormName()] = ['#type' => 'hidden', '#value' => 1];

The form value is not used by ConfirmFormBase's submit handler, although one subclass in core, UserMultipleCancelConfirm does check it.

However, because this form element uses the #value property, its value will ALWAYS be 1 according to the docs for Drupal\Core\Render\Element\Hidden:

 * - #value: The value of the form element. The Form API ensures that this
 *   value remains unchanged by the browser.

Therefore, AFAICT it doesn't serve any purpose and both the element and the ConfirmFormInterface::getFormName() could be removed.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet