mtd: rawnand: remove redundant assignment to variable bbtblocks

The variable bbtblocks is being assigned a value that is never
read. The assignment is redundant and can be removed.

Cleans up clang scan build warning:
drivers/mtd/nand/raw/nand_bbt.c:579:3: warning: Value stored to
'bbtblocks' is never read [deadcode.DeadStores]

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20240209174019.3933233-1-colin.i.king@gmail.com