nspawn: do not emit any warning when $UNIFIED_CGROUP_HIERARCHY is used

Initially I thought this is a good idea, but when reviewing a different PR
(https://github.com/systemd/systemd/pull/13862#discussion_r340604313) I changed
my mind about this. At some point we probably should start warning about the
old option name, and yet later remove it. But it'll make it easier for people
to transition to the new option name if there's a period of support for both
names without any fuss. There's nothing particularly wrong about the old name,
and there is no support cost.

Fixes #13919 (by avoiding the issue completely).