systemctl-start-unit: Subscribe() is unnecessary if we RefUnit explicitly

systemctl-start-unit: Subscribe() is unnecessary if we RefUnit explicitly

Subscribe() enables full signal delivery on API bus. But aside from
that, if a unit/job is explicitly Ref()'d, manager also emits the
signal. See bus_foreach_bus() for details.

bus-wait-for-units refs every unit to wait for, so there's no need
to Subscribe() on top of that. In verb_clean_or_freeze() Subscribe()
is not called either.