staging/xgifb: Remove unsupported mode LCD_320x480

This patch removes the probed mode LCD_320x480 which isn't supported
anyway since this mode falls through to the default (=invalid) mode in
the
XGIfb_validate_mode function (see line 529 ff. for details. the
commented out code for this mode is also removed).

By removing this assignment, we can use the LCD_TYPEs from the sis
driver without modifications.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>