[meta] Remove SimpleTest support

Problem/Motivation

SimpleTest was deprecated in Drupal 8 and moved to contrib in Drupal 9. There has been no activity in the contrib issue queue and it seems everyone has migrated to PHPUnit, which was the desired outcome.

Steps to reproduce

Proposed resolution

Remove all remnants of SimpleTest from core in Drupal 10.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet