Reasons for doing that:
1. it's consistent with other plugins.
2. it isn't exposed in the gui (user has to use xfconf-query tool
   to set it).
3. previous behavior was likely a workaround for lack of desbar mode
   (users which preferred to have the clock horizontal in a vertical
   panel by default are likely to use deskbar mode now).
4. horizontal labels don't fit in narrow vertical panels.

We could perhaps remove this property instead.