Problem/Motivation
SimpleTest is no longer useful, everyone has migrated to PHPUnit.
There is still support code for SimpleTest tests in TestDiscovery and TestRunnerKernel (and probably some related classes).
Steps to reproduce
Proposed resolution
Remove code relating to SimpleTest-based tests from these classes.
Deprecation is not required as tests are considered internal and users have had the full Drupal 9 cycle to upgrade to PHPUnit.
