Issue #3615914: Remove Drupal\domain\Form\DomainDeleteForm in 4.x

Removes Drupal\domain\Form\DomainDeleteForm and points the domain entity type's delete form handler at Drupal\Core\Entity\EntityDeleteForm.

The subclass existed only to override getCancelUrl() with the domain.admin route, because entity.domain.collection, the route name core derives from the collection link template, did not exist. #3613844 declared that name as an alias of the listing route, so the parent implementation now resolves the same listing page through $entity->toUrl('collection'). That also covers the post-deletion redirect, which EntityDeleteForm::submitForm() takes from getCancelUrl() as well.

DomainFormsTest submitted the delete form without asserting where it leads, so both targets are now asserted there.

This applies to 4.x only: route aliases require Drupal 11.2, while 3.x still supports Drupal 10, which is why #3553403 dropped the alias and added the form in the first place.