evb-rk3399: avoid redefinition of CONFIG_ENV_OFFSET

CONFIG_ENV_OFFSET is already defined in
include/configs/rockchip-common.h

For CONFIG_ROCKCHIP_BACK_TO_BROM=y we redefine CONFIG_ENV_OFFSET
to an unsuitable value. We were lucky to get a compiler warning.

Remove the incorrect redefinition.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>