arm: mx6: remove unused config variable CONFIG_SPL_NAND_MXS

The config variable CONFIG_SPL_NAND_MXS is only set in
include/configs/imx6_spl.h but used nowhere.
Remove it.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>