When transitioning to usrmerged systemd we introduced the %usrmerged macro.
Since there is no way back for Tumbleweed and already released distributions
won't ever go for usrmerge, the macro is actually obsolete. It's sufficient to
check for Tumbleweed which is usually done by checking %suse_version.

So the %usrmerged macro in spec files needs to be removed. Of course spec files
could manually set it based on %suse_version if that's convenient. Spec files
just shouldn't rely on an externally set %usrmerged anymore.