remove_salt: remove the repo file

The repo is now 404 which might prevent us from deleting it. Hence just
delete the file.

remove_salt.yml

 ---
 - hosts: all
   tasks:
+    - name: Remove saltstack repo file
+      file:
+        path: /etc/apt/sources.list.d/saltstack.list
+        state: absent
     - name: Remove saltstack repository
       apt_repository:
         repo: "{{ item }}"