[quickview] Remove unused variable |index| from |onQuickViewKeyDown_|

Bug: 1038799
Change-Id: Ic1254bd4b91d3d801f7021cfe157fd1e581d9d9e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2014219
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733957}

ui/file_manager/file_manager/foreground/js/quick_view_controller.js

@@ -173,7 +173,6 @@ class QuickViewController {
    */
   onQuickViewKeyDown_(event) {
     if (this.quickView_.isOpened()) {
-      let index;
       switch (event.key) {
         case ' ':
         case 'Escape':