mtd: rawnand: Get rid of the default ONFI timing mode

The ->choose_interface() hook is here for manufacturer drivers to
provide a better timing interface than the default one, this field is
not needed anymore.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/linux-mtd/20200529111322.7184-28-miquel.raynal@bootlin.com