SilcBool silc_schedule_task_del_by_callback(SilcSchedule schedule,
                                                SilcTaskCallback callback);

    Deletes a task from the scheduler by the specified `callback' task
    callback function.  Returns FALSE if such task with such callback
    does not exist.

    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.