Remove aufs-tools from Ubuntu requirement

aufs-tools was required for docker.io package originally,
but Kubespray installs docker-ce package instead today.
In addition, Ubuntu 20.04 doesn't provide aufs-tools as [1].
Then this removes aufs-tools from Ubuntu requirement.

[1]: https://bugs.launchpad.net/ubuntu/+source/aufs-tools/+bug/1947004

roles/kubernetes/preinstall/vars/ubuntu.yml

@@ -1,7 +1,6 @@
 ---
 required_pkgs:
   - python3-apt
-  - aufs-tools
   - apt-transport-https
   - software-properties-common
   - conntrack