btrfs: remove unused member err from reada_extent

Seems to be unused since the initial commit, we ignore readahead errors
anyway, the full read will handle that if necessary.
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/reada.c

@@ -66,7 +66,6 @@ struct reada_extctl {
 struct reada_extent {
 	u64			logical;
 	struct btrfs_key	top;
-	int			err;
 	struct list_head	extctl;
 	int 			refcnt;
 	spinlock_t		lock;