mtd: rawnand: Get rid of the ->read_word() hook

mtd: rawnand: Get rid of the ->read_word() hook

Commit 8548f1709fee ("mtd: nand: use read_oob() instead of cmdfunc()
for bad block check") removed this only user of the ->read_word()
method but kept the hook in place. Remove it now.

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