[BUG] Remove one-off (singleshot) task property

The one-off task property seems to be legacy. Both ForecastTask and PeriodicTask are somehow periodic distinguished exclusively by their corresponding schedule() method implementation. Meanwhile a PeriodicTask implements its schedule() method in a way that the next execution is based on the last execution time, the ForecastTask simply gathers the next forecast from the project's list of forecasts. The one-off property was not meaningful at all.