mtd: nand: Drop useless 'depends on' in Kconfig

Both OneNAND and raw NAND bits can't be compiled if MTD is disabled
because of the if/endif logic in drivers/mtd/Kconfig. There is no need
for an extra "depends on MTD" in their respective Kconfig files.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/linux-mtd/20200529002517.3546-12-miquel.raynal@bootlin.com