mtd: rawnand: diskonchip: Get rid of the legacy interface implementation

Now that exec_op() has been implemented we can get rid of the legacy
interface implementation.
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200501143917.1388957-6-boris.brezillon@collabora.com