mtd: rawnand: vf610_nfc: remove old hooks

Now that the driver is using ->exec_op(), remove the old
hooks.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>