mtd: nftl: remove redundant variable nb_erases

Variable nb_erases is being assigned but is never used hence it is
redundant and can be removed.

Cleans up clang warning:
warning: variable 'nb_erases' set but not used [-Wunused-but-set-variable]

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>