ARM: shmobile: defconfig: Remove MACH_GENMAI

The CONFIG_MACH_GENMAI is scheduled for removal so remove it from
shmobile_defconfig.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
[horms+renesas@verge.net.au: revised changelog for updated commit order]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>