Simplify the ART Buildbot clean-up script regarding chroot support.

The ART Buildbot is now executing the "setup device" step after the
"device pre-run cleanup" step, and the "tear down device" step before
the "device post-run cleanup" step, so we no longer need to preserve
any part of the chroot directory when cleaning up. Therefore, try to
remove the entire chroot directory during cleanup.

Test: Rely on the ART Buildbot
Bug: 34729697
Change-Id: I6a8c2017bc81f6cb5ed1781d2f97c0169d803532