sisfb: delete dead SIS_XORG_XF86 code

Delete code for compiling the driver for X.org/XFree86. The development
has forked, so there is no point keeping this code in the tree.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: Thomas Winischhofer <thomas@winischhofer.net>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>