mtd: onenand: kill unused variable

Fix this gcc warning:

drivers/mtd/onenand/onenand_base.c: In function ‘onenand_block_markbad’:
drivers/mtd/onenand/onenand_base.c:2636:23: warning: unused variable ‘this’ [-Wunused-variable]
Signed-off-by: NArtem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: NDavid Woodhouse <David.Woodhouse@intel.com>

drivers/mtd/onenand/onenand_base.c

@@ -2633,7 +2633,6 @@ static int onenand_default_block_markbad(struct mtd_info *mtd, loff_t ofs)
  */
 static int onenand_block_markbad(struct mtd_info *mtd, loff_t ofs)
 {
-	struct onenand_chip *this = mtd->priv;
 	int ret;
 
 	ret = onenand_block_isbad(mtd, ofs);