mtd: rawnand: sunxi: Stop supporting ECC_HW_SYNDROME mode

This mode is not used by any existing setup and should not be used
because it overwrites the BBMs. Let's just remove it before someone
starts using it.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Tested-by: Miquel Raynal <miquel.raynal@bootlin.com>