[NbtkWidget] remove the tooltip from the parent, rather than unparent
[NbtkWidget] remove the tooltip from the parent, rather than unparent

This ensures the parent (which should be the stage) is notified that the
actor is removed.

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