It's not a portable API, so remove it. The corresponding backend
specific functions are still available, if they were implemented, e.g.
gdk_macos_monitor_get_workarea() and gdk_x11_monitor_get_workarea().
The migration from GdkScreen's monitor API to GdkMonitor left out a way
to get the connector's name of a monitor.
While there's no real guarantee coming from the underlying graphics
system that the connector's name is stable, some system components may
use it to uniquely identify a monitor until the next plug in/out event.
