mtd: rawnand: Kill cafe_nand_bug()

Leaving a function pointer to NULL should be enough to trigger a NULL
pointer exception, and anyway, if we want to BUG() when some missing
hooks are called, this should be done in the core, so let's drop the
cafe_nand_bug() dummy function.

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