Purge all backup::client related packages/confs

Purge the amanda packages
Purge configuration files/directories
Remove dependent templates/source

When merging this also remove the following from the private repo:

* files/backup/ssh-keys/authorized_keys_client
* files/backup/ssh-keys/amanda_restore

Change-Id: I580f6e5237c21d859a831a442accad9fca2a1f3d