docs: Remove clutter_stage_get_default()

A lot of the example code in the cookbook and the API reference still
uses the default stage — sometimes as if it were a non-default one,
which once again demonstrates how the default stage was a flawed concept
that just confused people.

doc/cookbook/examples/events-pointer-motion-crossing.c

@@ -37,7 +37,7 @@ main (int argc, char *argv[])
   if (clutter_init (&argc, &argv) != CLUTTER_INIT_SUCCESS)
     return 1;
 
-  stage = clutter_stage_get_default ();
+  stage = clutter_stage_new ();
   clutter_stage_set_color (CLUTTER_STAGE (stage), &stage_color);
   clutter_stage_set_title (CLUTTER_STAGE (stage), "btn");