nspawn: drop sd_booted() check

nspawn: drop sd_booted() check

We have no such check in any of the other tools, hence don't have one in
nspawn either.

(This should make things nicer for Rocket, among other things)

Note: removing this check does not mean that we support running nspawn
on non-systemd. We explicitly don't. It just means that we remove the
check for running it like that. You are still on your own if you do...