md: remove set but not used variable 'bi_rdev'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/md/md.c: In function 'md_integrity_add_rdev':
drivers/md/md.c:2149:24: warning:
 variable 'bi_rdev' set but not used [-Wunused-but-set-variable]

It not used any more after commit
  1501efad ("md/raid: only permit hot-add of compatible integrity profiles")
Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Signed-off-by: Shaohua Li <shli@fb.com>

drivers/md/md.c

@@ -2148,14 +2148,12 @@ EXPORT_SYMBOL(md_integrity_register);
  */
 int md_integrity_add_rdev(struct md_rdev *rdev, struct mddev *mddev)
 {
-	struct blk_integrity *bi_rdev;
 	struct blk_integrity *bi_mddev;
 	char name[BDEVNAME_SIZE];
 
 	if (!mddev->gendisk)
 		return 0;
 
-	bi_rdev = bdev_get_integrity(rdev->bdev);
 	bi_mddev = blk_get_integrity(mddev->gendisk);
 
 	if (!bi_mddev) /* nothing to do */