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

Now that exec_op() is implemented, we can get rid of all the legacy
hooks.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Tested-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200720131356.1579073-7-tudor.ambarus@microchip.com