postfix: Remove dependency

We'll track those manually in the playbook for now.
Signed-off-by: Florian Pritz <bluewind@xinu.at>

roles/postfix/tasks/main.yml

 ---
 
-dependencies:
-  # TODO does this work with the dkim_selector variable used in opendkim?
-  - {role: "opendkim" }
-
 - name: install postfix
   pacman: name=postfix state=present