Revert "services.xserver: assert that either desktop- or window manager is not "none""

This reverts commit 93c54acf.
This reopens #30517 @nbp @Ma27

Breaking people's config for this is hardly reasonable as is. If it
absolutely cannot be avoided, at the very least, we need to provide
clear instructions on what people need to upgrade in their config. I
actually had to bisect to the commit, to even find out what property I
should change or define, as the error message was useless. It didn't
even mention a property name.

Discussion on the PR seems to be ongoing, so I'm reverting this, so we
don't break people's systems on unstable.