nixos: don't white-list port 631 in the firewall when CUPS is enabled

If you want CUPS to receive UDP printer announcements from the rest of the
world, please add

  networking.firewall.allowedUDPPorts = [ 631 ];

to /etc/nixos/configuration.nix.

See <http://lists.science.uu.nl/pipermail/nix-dev/2013-November/011997.html>
for the discussion that lead to this.

nixos/modules/services/printing/cupsd.nix

@@ -223,9 +223,6 @@ in
         </Policy>
       '';
 
-    # Allow CUPS to receive IPP printer announcements via UDP.
-    networking.firewall.allowedUDPPorts = [ 631 ];
-
     security.pam.services.cups = {};
 
   };