include: linux: mtd: Remove duplicate include of nand.h

linux/mtd/nand.h has been included at line 17.
So we remove the duplicate one at line 21.

Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210323031737.259365-1-wanjiabing@vivo.com