staging: xgifb: delete offscreen memory management

The offscreen memory area currently conflicts with the video memory
exported to the framebuffer layer. The driver does not utilize offscreen
memory, so the functionality can be deleted.

The patch also eliminates the one last memory leak when the driver
is unloaded.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>