perf: #3593202 Drop the explicit clear of plugin caches in drupal_flush_all_caches()

  // Rebuild all information based on new module data.

  \Drupal::moduleHandler()->invokeAll('rebuild');

  // Clear all plugin caches.

  \Drupal::service('plugin.cache_clearer')->clearCachedDefinitions();

  // Rebuild the menu router based on all rebuilt data.

  // Important: This rebuild must happen last, so the menu router is guaranteed

  // to be based on up to date information.