containers: Remove check for var/lib/startup-done.

Fixes a leftover from 330fadb7060074bd5c38e5dcefc394abaf8d6a09.

We're using systemd dbus notifications now and this leftover caused the
startup notification to fail.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
containers: Don't descend into mounts on destroy.

This tells the sad tale of @the-kenny who had bind-mounted his home
directory into a container. After doing `nixos-container destroy` he
discovered that his home directory went from "full of precious data" to
"no more data".

We want to avoid having similar sad tales in the future, so this now also
check this in the containers VM test.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>