staging: xgifb: delete the driver

There has not been any real work done on cleaning this driver up and
getting it out of the staging tree in years.  Also, no new fb drivers
are being added to the tree, so it should be converted into a drm driver
as well.

Due to the lack of interest in this codebase, just drop it.

Cc: Arnaud Patard <arnaud.patard@rtp-net.org>
Cc: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Reported-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>