displaymanager: Emit display-opened directly

Instead of letting every backend do it manually.

gdk/quartz/gdkdisplay-quartz.c

@@ -136,9 +136,6 @@ _gdk_quartz_display_open (const gchar *display_name)
 
   g_signal_emit_by_name (_gdk_display, "opened");
 
-  g_signal_emit_by_name (gdk_display_manager_get (),
-			 "display_opened", _gdk_display);
-
   return _gdk_display;
 }