testsuite: Fix broken test to not unref surfaces twice

The test doesn't hold 2 references, it holds only one.

The reason one unref can cause a leak is that some backends - like X11 -
only destroy the surface once the DestroyNotify event from the X server
has come in.

testsuite/gtk/shortcuts.c

@@ -360,7 +360,6 @@ test_trigger_trigger (void)
     }
 
   gdk_surface_destroy (surface);
-  g_object_unref (surface);
 
   g_object_unref (trigger[0]);
   g_object_unref (trigger[1]);