Absent sources.list on noble

On a fresh install, this is an empty file with a comment pointing to
/etc/apt/sources.list.d/ubuntu.sources and isn't needed.

In the upgrade case it doesn't make sense for us to create the empty
pointer file, so let's just ensure it's absent entirely.

install_files/ansible-base/roles/common/tasks/apt_sources.yml

@@ -20,6 +20,18 @@
   tags:
     - apt
 
+# Remove the obsolete sources.list on noble. On a fresh install,
+# this is an empty file with a comment pointing to ubuntu.sources.
+# On upgrade from focal, we're deleting this file.
+- name: Remove obsolete sources.list (noble)
+  template:
+    dest: /etc/apt/sources.list
+    state: absent
+  when: ansible_distribution_release != "focal"
+  notify: update apt cache
+  tags:
+    - apt
+
 # Ensure apt cache is updated before proceeding, otherwise
 # packages may fail to install.
 - meta: flush_handlers