interfaces: remove use of adv_dhcp6_prefix_interface_statement_sla_id

Section of code I've removed had been removed previously as during recent
PR's the ability to iterate through the interfaces was added. This 'solo'
bit of code appears to have made its way back in, don't know where and
don't know when as the song goes. Remove SLA_ID from Prefix interface.
It's collected from the LAN settings

src/etc/inc/interfaces.inc

@@ -3113,17 +3113,6 @@ function DHCP6_Config_File_Advanced($interface, $wancfg, $wanif)
             $id_assoc_statement_prefix .= ";\n";
         }
 
-        if (is_numeric($wancfg['adv_dhcp6_prefix_interface_statement_sla_id'])) {
-            $id_assoc_statement_prefix .= "  prefix-interface {$wanif} {\n";
-            $id_assoc_statement_prefix .= "    sla-id {$wancfg['adv_dhcp6_prefix_interface_statement_sla_id']};\n";
-            if (($wancfg['adv_dhcp6_prefix_interface_statement_sla_len'] >= 0) &&
-                ($wancfg['adv_dhcp6_prefix_interface_statement_sla_len'] <= 128)
-            ) {
-                $id_assoc_statement_prefix .= "    sla-len {$wancfg['adv_dhcp6_prefix_interface_statement_sla_len']};\n";
-            }
-            $id_assoc_statement_prefix .= "  };\n";
-        }
-
         $iflist = link_interface_to_track6($interface);
 
         foreach ($iflist as $friendly => $ifcfg) {

src/opnsense/service/tests/config/config.xml

@@ -40,7 +40,6 @@
       <adv_dhcp6_id_assoc_statement_prefix_id/>
       <adv_dhcp6_id_assoc_statement_prefix_pltime/>
       <adv_dhcp6_id_assoc_statement_prefix_vltime/>
-      <adv_dhcp6_prefix_interface_statement_sla_id/>
       <adv_dhcp6_prefix_interface_statement_sla_len/>
       <adv_dhcp6_authentication_statement_authname/>
       <adv_dhcp6_authentication_statement_protocol/>

src/www/interfaces.php

@@ -343,7 +343,7 @@ function get_wireless_channel_info($interface) {
       "adv_dhcp6_id_assoc_statement_address_pltime", "adv_dhcp6_id_assoc_statement_address_vltime",
       "adv_dhcp6_id_assoc_statement_prefix_enable", "adv_dhcp6_id_assoc_statement_prefix", "adv_dhcp6_id_assoc_statement_prefix_id",
       "adv_dhcp6_id_assoc_statement_prefix_pltime", "adv_dhcp6_id_assoc_statement_prefix_vltime",
-      "adv_dhcp6_prefix_interface_statement_sla_id", "adv_dhcp6_prefix_interface_statement_sla_len",
+      "adv_dhcp6_prefix_interface_statement_sla_len",
       "adv_dhcp6_authentication_statement_authname", "adv_dhcp6_authentication_statement_protocol", "adv_dhcp6_authentication_statement_algorithm",
       "adv_dhcp6_authentication_statement_rdm", "adv_dhcp6_key_info_statement_keyname", "adv_dhcp6_key_info_statement_realm",
       "adv_dhcp6_key_info_statement_keyid", "adv_dhcp6_key_info_statement_secret", "adv_dhcp6_key_info_statement_expire",
@@ -1110,7 +1110,6 @@ interface_sync_wireless_clones($a_interfaces[$if], false);
                     $new_config['adv_dhcp6_id_assoc_statement_prefix_id'] = $pconfig['adv_dhcp6_id_assoc_statement_prefix_id'];
                     $new_config['adv_dhcp6_id_assoc_statement_prefix_pltime'] = $pconfig['adv_dhcp6_id_assoc_statement_prefix_pltime'];
                     $new_config['adv_dhcp6_id_assoc_statement_prefix_vltime'] = $pconfig['adv_dhcp6_id_assoc_statement_prefix_vltime'];
-                    $new_config['adv_dhcp6_prefix_interface_statement_sla_id'] = $pconfig['adv_dhcp6_prefix_interface_statement_sla_id'];
                     $new_config['adv_dhcp6_prefix_interface_statement_sla_len'] = $pconfig['adv_dhcp6_prefix_interface_statement_sla_len'];
                     $new_config['adv_dhcp6_authentication_statement_authname'] = $pconfig['adv_dhcp6_authentication_statement_authname'];
                     $new_config['adv_dhcp6_authentication_statement_protocol'] = $pconfig['adv_dhcp6_authentication_statement_protocol'];
@@ -2678,8 +2677,6 @@ function toggle_allcfg() {
                           <td><i class="fa fa-info-circle text-muted"></i> <?=gettext("Prefix Interface");?></td>
                           <td>
                             <?=gettext("Prefix Interface "); ?>
-                            <i><?=gettext("Site-Level Aggregation Identifier"); ?></i>
-                            <input name="adv_dhcp6_prefix_interface_statement_sla_id" type="text" id="adv_dhcp6_prefix_interface_statement_sla_id" value="<?=$pconfig['adv_dhcp6_prefix_interface_statement_sla_id'];?>" />
                             <i><?=gettext("Site-Level Aggregation Length"); ?></i>
                             <input name="adv_dhcp6_prefix_interface_statement_sla_len" type="text" id="adv_dhcp6_prefix_interface_statement_sla_len" value="<?=$pconfig['adv_dhcp6_prefix_interface_statement_sla_len'];?>" />
                           </td>