ARM: dts: zte: remove num-slots from zx296702-ad1

dwmmc driver deprecated num-slots and plan to get rid
of it finally. Just move a step to cleanup it from DT.
Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Reviewed-by: Jun Nie <jun.nie@linaro.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/zx296702-ad1.dts

@@ -18,7 +18,6 @@ memory {
 };
 
 &mmc0 {
-	num-slots = <1>;
 	supports-highspeed;
 	non-removable;
 	disable-wp;
@@ -31,7 +30,6 @@ slot@0 {
 };
 
 &mmc1 {
-	num-slots = <1>;
 	supports-highspeed;
 	non-removable;
 	disable-wp;