make_chroot: drop buildbot arg hack

With our migration to build API and off of buildbot, this hack from
2010 shouldn't be necessary anymore.

BUG=None
TEST=CQ passes

Change-Id: Ic16b071ae1af52ce6987bc6afd066c7d06ac70a0
Reviewed-on: https://chromium-review.googlesource.com/1722402
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Alex Klein <saklein@chromium.org>
2 files changed