[style] remove references to NbtkScrollButton
[style] remove references to NbtkScrollButton

NbtkScrollButton is no longer used for steppers in scroll bars

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