atomic-host: Drop writing /etc/sysconfig/firstboot
We don't include firstboot in AH, we use cloud-init, so nothing
is ever going to parse this. Drop it, since it shows up as a delta
in `ostree admin config-diff`, and further we want to reduce the
amount of stuff in this ks.
Signed-off-by: Colin Walters <walters@verbum.org>