Remove warning "Couldn't export handle, unsupported windowing system"

For GDK_WINDOWING_BROADWAY, GDK_WINDOWING_WIN32 and GDK_WINDOWING_QUARTZ
links open correctly. For example, for GDK_WINDOWING_WIN32 this happens
via gspawn-win32-helper.exe / gspawn-win32-helper-console.exe. If these
helpers are missing, a corresponding error message is displayed, see
e.g. gtk3-demo.exe => Links. So there are already other error messages
if something goes wrong. For gtk4 the behavior also occurs without
warning. In my view this warning is more confusing than helpful.

gtk/gtkwindow.c

@@ -12989,8 +12989,6 @@ gtk_window_export_handle (GtkWindow               *window,
     }
 #endif
 
-  g_warning ("Couldn't export handle, unsupported windowing system");
-
   return FALSE;
 }