MAINTAINERS: Remove my name from the MTD and NAND entries

I am less and less active on the MTD/NAND fronts, and I don't think
it will get any better in the future. Let's remove my name so that
people don't expect me to review/merge their patches.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Richard Weinberger <richard@nod.at>