role(postfix): remove export-grade dh params

According to upstream documentation[0], the configuration parameter
smtpd_tls_dh512_param_file will be ignored from the next release of
Postfix (3.6).

[0]: http://www.postfix.org/postconf.5.html#smtpd_tls_dh512_param_file

roles/postfix/tasks/main.yml

@@ -39,11 +39,6 @@
   notify:
     - reload postfix
 
-- name: create dhparam 512
-  command: openssl dhparam -out /etc/postfix/dh_512.pem 512 creates=/etc/postfix/dh_512.pem
-  notify:
-    - reload postfix
-
 - name: install postfix cert renewal hook
   template: src=letsencrypt.hook.d.j2 dest=/etc/letsencrypt/hook.d/postfix owner=root group=root mode=0755
   when: postfix_smtpd_public

roles/postfix/templates/main.cf.j2

@@ -20,7 +20,6 @@ smtpd_tls_key_file = /etc/letsencrypt/live/{{inventory_hostname}}/privkey.pem
 {% endif %}
 
 smtpd_tls_dh1024_param_file = /etc/postfix/dh2048.pem
-smtpd_tls_dh512_param_file = /etc/postfix/dh_512.pem
 smtpd_tls_eecdh_grade = ultra
 tls_preempt_cipherlist = yes
 smtpd_tls_loglevel = 1