MT#6731 Remove "back" param from form if posted.

Somehow this sneaked in from somewhere and so far only happens with
resellers, so make sure to just clear it if a form got posted.

lib/NGCP/Panel/Utils/Navigation.pm

@@ -76,6 +76,8 @@ sub check_form_buttons {
             $c->detach;
         }
     }
+    delete $form->params->{back} if $posted;
+    delete $form->values->{back} if $posted;
 }
 
 sub back_or {