ARM: shmobile: Remove Koelsch reference DTS

Now that the DTS file r8a7790-koelsch.dts can be used with
board-koelsch.c and board-koelsch-reference.c, proceed with removing
r8a7790-koelsch-reference.dts.
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>

arch/arm/boot/dts/Makefile

@@ -225,7 +225,6 @@ dtb-$(CONFIG_ARCH_SHMOBILE_LEGACY) += emev2-kzm9d.dtb \
 	r8a7779-marzen.dtb \
 	r8a7779-marzen-reference.dtb \
 	r8a7791-koelsch.dtb \
-	r8a7791-koelsch-reference.dtb \
 	r8a7790-lager.dtb \
 	r8a7790-lager-reference.dtb \
 	sh73a0-kzm9g.dtb \