Don't generate the clamav configuration if ANTIVIRUS is none.

Otherwise the rspamd will try to connect to none, timing out several times and
leading to poor performance.

services/rspamd/conf/antivirus.conf

@@ -1,6 +1,8 @@
+{% if ANTIVIRUS == 'clamav' %}
 clamav {
   attachments_only = true;
   symbol = "CLAM_VIRUS";
   type = "clamav";
   servers = "antivirus:3310";
 }
+{% endif %}