RFC: remove aufs storage driver

[Luap99]

AFAIK aufs has never been upstreamed and from a quick search I could not find any distro who still supports it.

Do we know anyone using this storage driver still? Looking at the git log and issue I cannot really find any references pointing at users.Most changes seem to comes from global changes to the driver interface.
Given that it seems like an unnecessary maintenance burden and I think it is time to remove it. From what I can see aufs hasn't been tested in over 4 years in upstream CI since containers/storage@a54d4e3

Is there a reason to keep it around?

[Luap99]

FWIW, docker removed it over two years ago moby/moby@3903f16 so I guess we really don't need it

[mtrmac]

Makes sense to me.

@nalind @mheon @giuseppe WDYT?

[giuseppe]

yeah, let's remove it

[nalind]

AFAICT the last Ubuntu LTS to include aufs was 20.04.

[Luap99]

AFAICT the last Ubuntu LTS to include aufs was 20.04.

That matches the commit which removed the CI tests so that lines up.

PR to drop it: #343