Remove $backupStaticAttributes.

Closes #3497482

core/tests/Drupal/KernelTests/KernelTestBase.php

@@ -104,15 +104,6 @@ abstract class KernelTestBase extends TestCase implements ServiceProviderInterfa
   use ProphecyTrait;
   use ExpectDeprecationTrait;
 
-  /**
-   * {@inheritdoc}
-   *
-   * Back up and restore static class properties that may be changed by tests.
-   *
-   * @see self::runTestInSeparateProcess
-   */
-  protected $backupStaticAttributes = TRUE;
-
   /**
    * {@inheritdoc}
    */
@@ -121,23 +112,6 @@ public function __construct(string $name) {
     $this->setRunTestInSeparateProcess(TRUE);
   }
 
-  /**
-   * {@inheritdoc}
-   *
-   * Contains a few static class properties for performance.
-   */
-  protected $backupStaticAttributesBlacklist = [
-    // Ignore static discovery/parser caches to speed up tests.
-    'Drupal\Component\Discovery\YamlDiscovery' => ['parsedFiles'],
-    'Drupal\Core\DependencyInjection\YamlFileLoader' => ['yaml'],
-    'Drupal\Core\Extension\ExtensionDiscovery' => ['files'],
-    'Drupal\Core\Extension\InfoParser' => ['parsedInfos'],
-    // Drupal::$container cannot be serialized.
-    'Drupal' => ['container'],
-    // Settings cannot be serialized.
-    'Drupal\Core\Site\Settings' => ['instance'],
-  ];
-
   /**
    * @var \Composer\Autoload\Classloader
    */