cros: Remove empty app_list directory

Seems un-used.

Change-Id: Iab2b42bdc5af8d8a37d33123ce50081b66fc5eb1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1874630Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708379}

chrome/browser/ui/ash/app_list/OWNERS

-calamity@chromium.org
-khmel@chromium.org
-xiyuan@chromium.org