btrfs: reada, remove pointless BUG_ON check for fs_info

We dereference fs_info several times, besides that post-mount functions
should never see a NULL fs_info.
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/reada.c

@@ -439,7 +439,6 @@ static struct reada_extent *reada_find_extent(struct btrfs_root *root,
 				/* ignore whether the entry was inserted */
 				radix_tree_delete(&dev->reada_extents, index);
 			}
-			BUG_ON(fs_info == NULL);
 			radix_tree_delete(&fs_info->reada_tree, index);
 			spin_unlock(&fs_info->reada_lock);
 			btrfs_dev_replace_unlock(&fs_info->dev_replace, 0);