mtd: nand: Remove useless line in Kconfig

Prepare changes that will lay in this file to better express what is
NAND related and what is not in menuconfig.
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>

drivers/mtd/nand/Kconfig

@@ -2,6 +2,5 @@ config MTD_NAND_CORE
 	tristate
 
 source "drivers/mtd/nand/onenand/Kconfig"
-
 source "drivers/mtd/nand/raw/Kconfig"
 source "drivers/mtd/nand/spi/Kconfig"