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>