Simplify & document unregister

1) Simplify the unregister task, which doesn't require any parameter,
   easier to maintain and less error prone.

2) Document with a playbook example in README how to unregister.

3) Remove useless register, never used later.

Change-Id: Ic1b6674c4608b555f51aa8c6716630c8d96b7fe2

README.md

@@ -103,6 +103,18 @@ Example Playbook with Satellite 6
       roles:
         - openstack.redhat-subscription
 
+Example Playbook to unregister
+------------------------------
+
+::
+
+    - hosts: all
+      tasks:
+        - name: Unregister the node
+          include_role:
+            name: openstack.redhat-subscription
+            tasks_from: unregister
+
 License
 -------
 

tasks/unregister.yml

@@ -1,12 +1,5 @@
 - name: Unregister Red Hat subscription
   redhat_subscription:
-    username: "{{ rhsm_username | default(omit) }}"
-    password: "{{ rhsm_password | default(omit) }}"
-    activationkey: "{{ rhsm_activation_key | default(omit) }}"
-    org_id: "{{ rhsm_org_id | default(omit) }}"
-    pool_ids: "{{ rhsm_pool_ids | default(omit) }}"
     state: "absent"
-    rhsm_baseurl: "{{ rhsm_baseurl | default(omit) }}"
-  register: subscrition_result
   tags:
     - rhsm_unregister