bus-unit-util: don't validate partition designator client side

When we parse properties set via "systemctl set-property" we should
validate to the point where the data we pass over the bus makes rough
sense, but we shouldn't needlessly check whether specified enum values
are among the known enum values. The server side checks that anyway
again, and it's kinda nice if an older systemctl can be used to talk to
a newer systemd.