systemd: drop machine UUID generation for unit file

systemd: drop machine UUID generation for unit file

To optimize startup a bit, drop machine UUID generation from the unit
file. Since a while D-Bus was already capable of making use of the
/etc/machine-id file, which is guaranteed to exist on systemd systems
(since the first released versions already), hence there's no need to
ever generate a D-Bus specific machine ID.

In times where userspace takes < 800ms to boot up getting rid of this
process optimizes boot time further.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=45520
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>