Remove !ci-backup play

Let's abandon the idea that we'll treat the backup server specially.
As long as we allow *any* automated remote access via ansible, we
have opened the door to potential compromise of the backup systems
if bridge is compromised.  Rather than pretending that this separation
gives us any benefit, remove it.

Change-Id: I751060dc05918c440374e80ffb483d948f048f36

playbooks/base.yaml

@@ -12,10 +12,6 @@
     - base-server
     - timezone
     - unbound
-
-- hosts: "!ci-backup:!disabled"
-  name: "Base: set up common environment on all hosts (except backup)"
-  roles:
     - exim
     - iptables
     - snmpd