mtd: nand: stop exporting nand_default_bbt

I removed the last non-nand_base users of this, and we shouldn't have
any more modules that need to access it. It's only non-static to share
between nand_base and nand_bbt.
Signed-off-by: Brian Norris <computersforpeace@gmail.com>

drivers/mtd/nand/nand_bbt.c

@@ -1392,4 +1392,3 @@ int nand_markbad_bbt(struct mtd_info *mtd, loff_t offs)
 }
 
 EXPORT_SYMBOL(nand_scan_bbt);
-EXPORT_SYMBOL(nand_default_bbt);