Clean up old rolling forecasting code after fixed-point migration

[BelhsanHmida]

Description

We moved to fixed-point forecasting, but parts of the old rolling forecasting infrastructure are still present and partially used.

Notably:

• make_fixed_viewpoint_forecasting() still raises NotImplementedError
• create_forecasting_jobs() calls make_rolling_viewpoint_forecasts()
• create_forecasting_jobs() is used in multiple tests
• handle_forecasting_exception() calls make_rolling_viewpoint_forecasts() as a fallback
• Utilities like forecast_horizons_for() and supported_horizons() are still present
• tests: test_forecasting_jobs.py, test_forecasting_jobs_fresh_db.py

Now we want to remove old rolling viewpoint forecasting as it's not supported in flexmeasures.

Question

• what should replace the current fallback in handle_forecasting_exception()?
  • Retry fixed-point?
  • Mark job as failed?
  • Implement a new fallback mechanism?