mtd: rawnand: gpmi: remove unused variable

The "private" member of struct gpmi_nand_data isn't used anywhere.
Remove it.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>