gir: Don't annotate user_data params with closure

The (closure) annotation should go on the callback parameter, e.g.

 * @callback: (scope async) (closure user_data): a callback to call when the operation is complete

However, heuristic number 378 picks up parameters named user_data automatically as the callback's user data if there is a callback.

cc: @ebassi could you please double check that this is all accurate?