systemctl: drop one must_be_root_check()

systemctl: drop one must_be_root_check()

(before)
$ build/systemctl list-machines
Need to be root.
$ sudo build/systemctl list-machines
NAME          STATE   FAILED JOBS
krowka (host) running      0    0
rawhide       running      0    0

2 machines listed.

(after)
$ build/systemctl list-machines
NAME          STATE   FAILED JOBS
krowka (host) running      0    0
rawhide       n/a          0    0

2 machines listed.

The output for non-root is missing some bits of information, but we display
what information is missing nicely, and e.g. in the case when no machines are
running at all, or only VMs are running, the unprivileged output would be the
same as the privileged one.