Remove purge_apt_sources

This was a setting added for infra cloud that had to do with bootstrap
order. It seems to have been cargo-culted elsewhere. Remove it. Let's be
specific with our sources.list files.

Change-Id: Iefbd59ad20e9fdc450d9a0c4e58b9cf4a89ff5a3

playbooks/group_vars/ci-backup.yaml

@@ -1,2 +0,0 @@
-# TODO(mordred) Why is this set?
-purge_apt_sources: false

playbooks/group_vars/mailman.yaml

@@ -2,5 +2,3 @@ exim_queue_interval: '1m'
 exim_queue_run_max: '50'
 exim_smtp_accept_max: '100'
 exim_smtp_accept_max_per_host: '10'
-# TODO(mordred) Why is this set?
-purge_apt_sources: false

playbooks/roles/base-repos/defaults/main.yaml

@@ -1 +0,0 @@
-purge_apt_sources: true

playbooks/roles/base-repos/tasks/Debian.yaml

@@ -11,7 +11,6 @@
     dest: /etc/apt/apt.conf.d/90no-translations
 
 - name: Replace sources.list file
-  when: purge_apt_sources
   copy:
     src: 'sources.list.{{ ansible_facts.lsb.codename }}.{{ ansible_facts.architecture }}'
     dest: /etc/apt/sources.list