Resolve "remove admin users feature"

Closes #1 (closed) drop support for admin_users feature. As previously said, this feature was system-related so it had nothing to do with yunohost role.

README.md

@@ -44,11 +44,6 @@ yunohost:
       firstname: John 
       lastname: Doe
       domain: '{{ extra_domain.[1] }}'
-  admin_users: 
-    - name: admin # user generated by yunohost as its admin user  
-      key: ssh-rsa ... # key for admin user 
-    - name: user1 # user generated by yunohost and defined by you  
-      key: ssh-rsa ... # key for  
 ```
 
 Dependencies