btrfs: Remove newest_gen argument from find_oldest_super_backup

It's no longer needed following cleanups around find_newest_backup_root

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>