Revert "block: Warn and free bio if bi_end_io is not set"

This reverts commit 95d4403889acbd98e06d41a255df76452210996a.

The patch is broken for on-stack bios, amongst other things.

fs/bio.c

@@ -1770,15 +1770,6 @@ void bio_endio(struct bio *bio, int error)
 		} else {
 			if (bio->bi_end_io)
 				bio->bi_end_io(bio, error);
-			else {
-				char dev_name[BDEVNAME_SIZE];
-
-				WARN(1, "bio_endio: bio for %s without endio\n",
-					bio->bi_bdev ? bdevname(bio->bi_bdev,
-					dev_name) : "(unknown)");
-				bio_put(bio);
-			}
-
 			bio = NULL;
 		}
 	}