ARM: shmobile: genmai: remove defconfig

The genmai board code is scheduled for removal and once
that occurs the defconfig will be of no use. Remove it.

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