mtd: nand_bbt: kill NAND_BBT_SCANALLPAGES

Now that the last user of NAND_BBT_SCANALLPAGES has been removed, let's
kill this peculiar BBT feature flag.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Reviewed-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>