systemctl: don't use get_process_comm() on non-local PIDs (#7518)

systemctl: don't use get_process_comm() on non-local PIDs (#7518)

Let's not use local process data for remote processes, that can only
show nonsense.

Maybe one day we should add a bus API to query the comm field of a
process remotely, but for now, let's not bother, the information is
redundant anyway, as the cgroup data shows it too (and the cgroup tree
is show as part of status as well, and is requested from remote through
dbus, without local kernel calls).

Fixes: #7516