dhcp: remove ::/64 magic as it uses AdvRouterAddr yes

We should restart when we need to these days, it messes with
virtual IP additions and it also breaks hashing for configuration
changes indicating there is no shift in prefix.

src/etc/inc/plugins.inc.d/dhcpd.inc

@@ -456,8 +456,6 @@ function dhcpd_radvd_configure($verbose = false, $blacklist = [])
         if ($autotype == 'slaac') {
             /* XXX this may be incorrect and needs an override or revisit */
             $networkv6 = '2000::/64';
-        } elseif (!is_subnetv6($networkv6)) {
-            $networkv6 = '::/64';
         }
 
         if (!empty(service_by_filter(['dns_ports' => '53']))) {