Removed a stale "AF_INET only" comment.

IPv6 client connections in mail modules have been supported since r2856.
diff --git a/src/mail/ngx_mail_handler.c b/src/mail/ngx_mail_handler.c
index a1c4464..ae955f9 100644
--- a/src/mail/ngx_mail_handler.c
+++ b/src/mail/ngx_mail_handler.c
@@ -38,8 +38,6 @@
 
     /* find the server configuration for the address:port */
 
-    /* AF_INET only */
-
     port = c->listening->servers;
 
     if (port->naddrs > 1) {