ARM: shmobile: dts: Remove r8a7791-koelsch-reference.dts

The dts file has been superseded by r8a7791-koelsch.dts and been removed
from the ARCH_SHMOBILE_LEGACY dtb target. Remove it.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>