mtd: rawnand: Kill the chip->scan_bbt() hook

None of the existing drivers are overloading the ->scan_bbt() method,
let's get rid of it and replace calls to ->scan_bbt() by
nand_create_bbt() ones.

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