systemctl: drop useless DBus calls from 'systemctl show foo.service'

systemctl: drop useless DBus calls from 'systemctl show foo.service'

systemctl called LoadUnit, GetUnit, GetAll in this order to get the properties.

It is useless to load units explicitly, because it won't ensure anything. The
unit may be freed immediately by the garbage collector.

It is unnecessary to call GetUnit, because systemctl can easily translate the
unit name to DBus path by itself.

GetAll will load the unit if necessary.