diff --git a/core/modules/simpletest/src/TestDiscovery.php b/core/modules/simpletest/src/TestDiscovery.php
index d5d7d8a576..a0be3dc140 100644
--- a/core/modules/simpletest/src/TestDiscovery.php
+++ b/core/modules/simpletest/src/TestDiscovery.php
@@ -25,13 +25,6 @@ class TestDiscovery {
   protected $classLoader;
 
   /**
-   * Backend for caching discovery results.
-   *
-   * @var \Drupal\Core\Cache\CacheBackendInterface
-   */
-  protected $cacheBackend;
-
-  /**
    * Cached map of all test namespaces to respective directories.
    *
    * @var array
@@ -70,14 +63,13 @@ class TestDiscovery {
    *   \Symfony\Component\ClassLoader\ApcClassLoader.
    * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
    *   The module handler.
-   * @param \Drupal\Core\Cache\CacheBackendInterface $cache_backend
-   *   (optional) Backend for caching discovery results.
+   * @param mixed $cache_backend
+   *   (optional) Unused.
    */
-  public function __construct($root, $class_loader, ModuleHandlerInterface $module_handler, CacheBackendInterface $cache_backend = NULL) {
+  public function __construct($root, $class_loader, ModuleHandlerInterface $module_handler, $cache_backend = NULL) {
     $this->root = $root;
     $this->classLoader = $class_loader;
     $this->moduleHandler = $module_handler;
-    $this->cacheBackend = $cache_backend;
   }
 
   /**
@@ -159,11 +151,6 @@ public function getTestClasses($extension = NULL, array $types = []) {
     $reader = new SimpleAnnotationReader();
     $reader->addNamespace('Drupal\\simpletest\\Annotation');
 
-    if (!isset($extension)) {
-      if ($this->cacheBackend && $cache = $this->cacheBackend->get('simpletest:discovery:classes')) {
-        return $cache->data;
-      }
-    }
     $list = [];
 
     $classmap = $this->findAllClassFiles($extension);
@@ -212,12 +199,6 @@ public function getTestClasses($extension = NULL, array $types = []) {
     // Allow modules extending core tests to disable originals.
     $this->moduleHandler->alter('simpletest', $list);
 
-    if (!isset($extension)) {
-      if ($this->cacheBackend) {
-        $this->cacheBackend->set('simpletest:discovery:classes', $list);
-      }
-    }
-
     if ($types) {
       $list = NestedArray::filter($list, function ($element) use ($types) {
         return !(is_array($element) && isset($element['type']) && !in_array($element['type'], $types));