shellDBus: Remove ShowMonitorLabels API
This API has been broken for quite some time now as the corresponding mutter function meta_monitor_manager_get_monitor_for_output was removed. If anyone tries to use it, we would just run into a backtrace.
This is safe to do for 3.32. For 3.34 it probably is a good idea to rename the ShowMonitorLabels2 API to ShowMonitorLabels and update gnome-control-center accordingly.