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

The genmai board now boots using the generic R7S72100 DT machine with
the same feature set as the board file. Remove the board file.

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>