Update security page to remove mention of deprecated mailing list

documentation/security.html

@@ -43,9 +43,7 @@ It is considered a safer behaviour not to disclose server information from your
 <p>
 Make sure you check <a href="http://support.chamilo.org/projects/chamilo-18/wiki/Security_issues">our security
     issues page</a> from time to time.
-Subscribe to our free security alerts mailing-list:
-    <a href="http://lists.chamilo.org/listinfo/security">http://lists.chamilo.org/listinfo/security</a> or that you
-   follow our security Twitter feed: <a href="http://twitter.com/chamilosecurity">http://twitter.com/chamilosecurity</a>.
+You can also follow our security Twitter feed: <a href="http://twitter.com/chamilosecurity">http://twitter.com/chamilosecurity</a>.
 </p>
 
 <h2><a name="3.Using-safe-browsers"></a>3. Using safe browsers</h2>