drop old chroot upgrade logic for /mnt/host paths

We changed paths to the source tree in Dec 2012 and added some hooks
to upgrade existing chroots.  We already dropped support for upgrading
pre-2015 chroots, so drop this logic too.

BUG=None
TEST=CQ passes

Change-Id: I8d1d2c05f319c40599c102c95199893f0e709041
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/1722401
Reviewed-by: Sloan Johnson <sloanjohnson@google.com>
Auto-Submit: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Sloan Johnson <sloanjohnson@google.com>
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
2 files changed