[nbtk-types] remove redundant enum
[nbtk-types] remove redundant enum

This enum is no longer used. Use of it was removed when NbtkButton was
re-factored to have the transition virtual function.

jonnylamb/mx,clutter-project/mx,clutter-project/mx,jonnylamb/mx,jonnylamb/mx,pwithnall/mx,clutter-project/mx,pwithnall/mx,pwithnall/mx,jonnylamb/mx,pwithnall/mx,clutter-project/mx