TestServiceProvider has unused code referring to Simpletest

Problem/Motivation

Discovered in #3203956: Fix UndefinedDocblockClass errors detected by Psalm:

core/tests/Drupal/KernelTests/TestServiceProvider.php:16:11:error - UndefinedDocblockClass: Docblock-defined class or interface Drupal\simpletest\TestBase does not exist

  /**
   * @var \Drupal\simpletest\TestBase
   */
  public static $currentTest;

TestServiceProvider::$currentTest is never set anywhere in core, so there is a bunch of dead code relating to this property.

Steps to reproduce

Proposed resolution

Remove the dead code.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet