Users must use luci-auth now.

Bug: 1001756
Change-Id: I04cab6bdbfbd958f386a4cab761dfe4d34073afc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1849810
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Anthony Polito <apolito@google.com>