x11: Simplify function

Displays get made default displays automatically, so there's no need to
do it in the vfunc.

gdk/x11/gdkdisplaymanager-x11.c

@@ -76,16 +76,7 @@ static GdkDisplay *
 gdk_x11_display_manager_open_display (GdkDisplayManager *manager,
                                       const gchar       *name)
 {
-  GdkDisplay *display;
-
-  display = _gdk_x11_display_open (name);
-  if (display != NULL)
-    {
-      if (gdk_display_manager_get_default_display (manager) == NULL)
-        gdk_display_manager_set_default_display (manager, display);
-    }
-
-  return display;
+  return _gdk_x11_display_open (name);
 }
 
 static void