gdataset: drop "Since" annotation from internal g_datalist_id_update_atomic()

This is internal API, and is only used via GLIB_PRIVATE_CALL(). Such
private API is not stable. That is, you must use the same version of
e.g. libgobject and libglib.

In that case, the "Since" annotation makes no sense.

glib/gdataset.c

@@ -1082,8 +1082,6 @@ g_datalist_id_remove_no_notify (GData	**datalist,
  * @user_data.
  *
  * Returns: the value returned by @callback.
- *
- * Since: 2.80
  */
 gpointer
 g_datalist_id_update_atomic (GData **datalist,