void silc_schedule_task_del_by_callback(SilcSchedule schedule,
                                            SilcTaskCallback callback);

    Deletes a task from the scheduler by the specified `callback' task
    callback function.

    It is safe to call this function in any place. Tasks may be removed
    in task callbacks (including in the task's own task callback) and
    in multi-threaded environment in other threads as well.