Don't attempt to cascade a delete from a view

The CombinedPredictionObservation model is actually not managed by
Django as it's a SQL view onto the database. When a Location is deleted
it shouldn't cascade and attempt to delete the
CombinedPredictionObservation.
Merge pull request #86 from sealevelresearch/dont-cascade-delete-from-view

Don't attempt to cascade a delete from a view