ARM: shmobile: r7s72100: genmai: Remove legacy board file

The genmai board now boots using DT and multiplatform kernel with the
same feature set as the legacy board. Remove the legacy board file and
the board Kconfig option.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Wolfram Sang <wsa@sang-engineering.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>