ARM: shmobile: Remove genmai_defconfig from MAINTAINERS

The genmai defconfig file has been removed by
3ed27bd9 ("ARM: shmobile: genmai: remove defconfig")
so remove its entry in the MAINTAINERS accordingly.

Reported-by: Joe Perches <joe@perches.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>