systemctl: stop saying "vendor preset"

systemctl: stop saying "vendor preset"

We have vendor presets, and local admin presets, and runtime presets
(under /usr/lib, /usr/local/lib and /etc, /run, respectively). When we
display preset state, it can be configured in any of those places, so
we shouldn't say anything about the origin.

(Another nice advantage is that it improves alignment:

[root@f36 ~]# systemctl list-unit-files multipathd.service
UNIT FILE          STATE   VENDOR PRESET
multipathd.service enabled enabled

^ this looks we have a "PRESET" column that is empty.)