arc-setup: Stop calling 'stop ui' in arc-system-mount.conf

With ag/3840877, push_to_device.py does that instead.

BUG=b:77494303
TEST=push_to_device.py still works

Change-Id: I750e16fd33d549433d9142c76dcbb0d9180d6ff7
Reviewed-on: https://chromium-review.googlesource.com/994384
Commit-Ready: Yusuke Sato <yusukes@chromium.org>
Tested-by: Yusuke Sato <yusukes@chromium.org>
Reviewed-by: Luis Hector Chavez <lhchavez@chromium.org>
diff --git a/arc/setup/etc/arc-system-mount.conf b/arc/setup/etc/arc-system-mount.conf
index 03de5bb..53fbdf8 100644
--- a/arc/setup/etc/arc-system-mount.conf
+++ b/arc/setup/etc/arc-system-mount.conf
@@ -31,8 +31,6 @@
   set +e -x
   . $ENVFILE
 
-  # Shut down the container.
-  stop ui
   # Unmount images.
   exec /usr/sbin/arc-setup --onetime-stop
 end script