Remove unused file.
Remove unused file.

Removes the unfinished and probably obsolete AccordionPane. Not included in the
build anyway.

https://bugzilla.gnome.org/show_bug.cgi?id=611960

petejohanson/hyena,dufoli/hyena,GNOME/hyena,arfbtwn/hyena,arfbtwn/hyena,GNOME/hyena,petejohanson/hyena,dufoli/hyena