Update to Drupal 8.0.0-rc3. For more information, see https://www.drupal.org/node/2608078

core/modules/editor/editor.libraries.yml

@@ -11,9 +11,6 @@ drupal.editor:
   version: VERSION
   js:
     js/editor.js: {}
-  css:
-    component:
-      css/editor.css: {}
   dependencies:
     - core/jquery
     - core/drupal

core/modules/editor/src/Form/EditorLinkDialog.php

@@ -53,13 +53,6 @@ class EditorLinkDialog extends FormBase {
       '#maxlength' => 2048,
     );
 
-    $form['attributes']['target'] = array(
-      '#title' => $this->t('Open in new window'),
-      '#type' => 'checkbox',
-      '#default_value' => !empty($input['target']),
-      '#return_value' => '_blank',
-    );
-
     $form['actions'] = array(
       '#type' => 'actions',
     );