Remove Drupal\domain\Form\DomainDeleteForm in 4.x

Problem/Motivation

#3553403: Use \Drupal\Core\Entity\EntityDeleteForm to delete a domain record left the DomainDeleteForm in place since the entity.domain.collection did not exist. The entity.domain.collection not exists, so the DomainDeleteForm no longer has a purpose.

Proposed resolution

Remove Drupal\domain\Form\DomainDeleteForm in 4.x and use Drupal\Core\Entity\EntityDeleteForm instead. This essentially reverts https://git.drupalcode.org/project/domain/-/merge_requests/256/diffs?com...

Remaining tasks

1. Write a merge request
2. Review
3. Commit

User interface changes

None

API changes

None

Data model changes

None

AI-Generated: Yes (Claude Code was used to help draft this issue summary and to write the code and tests on the merge request. I reviewed and ran the work myself before posting it.)