Remove ahbl as it's being winded down

http://ahbl.org/content/changes-ahbl

Fixes #232

roles/mailserver/templates/etc_postfix_main.cf.j2

   bl.spamcop.net*2
   dnsbl.sorbs.net*1
   spam.spamrats.com*2
-  dnsbl.ahbl.org*2
 postscreen_dnsbl_threshold = 3
 postscreen_dnsbl_action = enforce
 postscreen_greet_action = enforce