Docker aufs and devicemapper directories both exist
It looks like your docker used to use aufs as the storage backend but switched to devicemapper. (probably in result of upgrading the docker package)
If you (or your package manager) change the storage backend, docker will NOT convert existing containers and/or images to use the new storage backend. Thus, you'll see an empty list of containers and images. But the files will remain there. So it is safe to delete the aufs directory as long as you don't need the containers and images inside it.