MAINTAINERS: remove entry for deleted pxa3xx_nand driver

The driver pxa3xx_nand.c has been replaced everywhere by its rework
called marvell_nand.c so this entry can now be removed.

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