Do not call popover_set_relative with NULL

This gives a critical warning since that function expects a
valid widget.

https://bugzilla.gnome.org/show_bug.cgi?id=724407

gtk/gtkmenubutton.c

@@ -1097,7 +1097,6 @@ gtk_menu_button_set_popover (GtkMenuButton *menu_button,
       g_signal_handlers_disconnect_by_func (priv->popover,
                                             menu_deactivate_cb,
                                             menu_button);
-      gtk_popover_set_relative_to (GTK_POPOVER (priv->popover), NULL);
     }
 
   priv->popover = popover;