debxo: drop gnome ubifs images

Gnome barely fits in the 1GB XO-1 nand; when running dist-upgrade
inside of gnome, gconftool-2 easily eats up all 256MB of ram and
causes an OOM crash.  The gnome utilities quickly exhaust the XO-1s
ram, as well.  As such, gnome will no longer be supported for the
XO-1 nand.  Drop the ubifs config files accordingly.

It will remain supported for the XO-1.5, as well as larger SD/USB
devices on XO-1.  Don't expect me to care about memory usage,
though! :)

Signed-off-by: Andres Salomon <dilinger@lunge.queued.net>