mtd: nand: remove unused blockmask variable

This patch fix the following build warning:
drivers/mtd/nand/nand_base.c:2671:30: attention : variable ‘blockmask’ set but not used [-Wunused-but-set-variable]

Fixes: 0b4773fd

 ("mtd: nand: Drop unused cached programming support")
Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>