ARM: shmobile: r7s72100: Remove r7s72100-genmai.dtb for ARCH_SHMOBILE_LEGACY

Forgotten by commit ad8c3af8

 ("ARM: shmobile: r7s72100: Remove
legacy board support").

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>