Remove ClamAV

I'd like to stop ClamAV scanning and remove all traces of it from our
offsite backup machine because:

- the duplicity managed files on here are compressed and (in most cases)
  encrypted which makes scanning for viruses either slow or impossible
- it uses a lot of CPU and RAM on what is a relatively small machine
Merge pull request #80 from alphagov/remove_clamav

Remove ClamAV