mtd: nand: remove EXPORT_SYMBOL of nand_scan_bbt()

mtd: nand: remove EXPORT_SYMBOL of nand_scan_bbt()

Since commit f93161a7570c547ddd7325748b7a3f67afe54424 ("mtd: nand_bbt:
make nand_scan_bbt() static"), the nand_scan_bbt() function is marked
as static but is still exported using EXPORT_SYMBOL(), which doesn't
make much sense.

This commit gets rid of the useless EXPORT_SYMBOL.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>