mtd: rawnand: Get rid of the duplicate nand_chip forward declaration

There's already a forward declaration of nand_chip at the beginning of
the file. Get rid of this one.
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>

include/linux/mtd/rawnand.h

@@ -210,9 +210,6 @@ enum nand_ecc_algo {
 #define NAND_CI_CELLTYPE_MSK	0x0C
 #define NAND_CI_CELLTYPE_SHIFT	2
 
-/* Keep gcc happy */
-struct nand_chip;
-
 /* ONFI version bits */
 #define ONFI_VERSION_1_0		BIT(1)
 #define ONFI_VERSION_2_0		BIT(2)