[NbtkGtkFrame] remove unused variable
[NbtkGtkFrame] remove unused variable

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