mtd: nand: remove deprecated pxa3xx_nand driver

All board files and defconfig files have been moved to use the new
marvell_nand driver instead of pxa3xx_nand, so we can safely remove this
file now. People should use the new driver which is supposed to behave
exactly like the old one.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>