Merge branch '6518-remove-default-rbl-entry-for-spamhaus' into 'develop'

Resolve "Remove default RBL entry for Spamhaus"

Closes #6518

See merge request ispconfig/ispconfig3!1817

Author: Till Brehm

install/tpl/server.ini.master

@@ -58,7 +58,7 @@ mailbox_size_limit=0
 message_size_limit=0
 mailbox_soft_delete=0
 mailbox_quota_stats=y
-realtime_blackhole_list=zen.spamhaus.org
+realtime_blackhole_list=
 overquota_notify_threshold=90
 overquota_notify_admin=y
 overquota_notify_reseller=y