www: prepare to die! fix validation code now that issue is triggered

PR: https://forum.opnsense.org/index.php?topic=3824
(cherry picked from commit 5d238412)

src/www/guiconfig.inc

@@ -168,10 +168,7 @@ function do_input_validation($postdata, $reqdfields, $reqdfieldsn, &$input_error
     }
 
     for ($i = 0; $i < count($reqdfields); $i++) {
-        // why.. do you want to do this? we send data to validate and then validate something else....
-        // can't now for sure if any page uses this wrong behaviour, but it should die eventually.
-        // TODO: kill
-        if (empty($_POST[$reqdfields[$i]]) && empty($_REQUEST[$reqdfields[$i]])) {
+        if (empty($postdata[$reqdfields[$i]])) {
             $input_errors[] = sprintf(gettext("The field %s is required."), $reqdfieldsn[$i]);
         }
     }