[widget] Remove unused DnD child pointer
[widget] Remove unused DnD child pointer

The drag and drop child is referenced elsewhere so there's no need
to keep a reference on it.

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