droppable: Fix the incorrect removal of qdata when disabling
droppable: Fix the incorrect removal of qdata when disabling

The line was accidentally trying to remove the qdata from the droppable
object instead of the stage.

Fixes http://bugzilla.clutter-project.org/show_bug.cgi?id=2208

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