Remove BindingPool::list_actions()

The clutter_binding_pool_list_actions() was not implemented. The
utility of a call listing all the action names is also debatable:
all the functions related to the key bindings take the key symbol
and modifiers -- except the block_action() and unblock_action()
pair.

clutter/clutter-binding-pool.c

@@ -601,12 +601,6 @@ clutter_binding_pool_override_closure (ClutterBindingPool  *pool,
     }
 }
 
-gchar **
-clutter_binding_pool_list_actions (ClutterBindingPool *pool)
-{
-  return NULL;
-}
-
 /**
  * clutter_binding_pool_find_action:
  * @pool: a #ClutterBindingPool