WebUI user-manager: Remove supervised users related dead code.

This code should have been removed as part of
https://chromium-review.googlesource.com/c/chromium/src/+/1029952/

Bug: 902959,828890
Change-Id: Ib7e00a646c864bea6b741924d6c592197ff2863d
Reviewed-on: https://chromium-review.googlesource.com/c/1327227
Reviewed-by: Esmael El-Moslimany <aee@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606972}
3 files changed