Remove cache from HazardCurveGetterPerAsset

Currently the HazardCurveGetterPerAsset caches hazard curves at each location. However, as each instance of HazardCurveGetterPerAsset is responsible for getting hazard curves for a single taxonomy (that is a single IMT) and as it is unlikely to have more than one asset of the same taxonomy at the same site, the cache is seldom hit (actually it has never been hit in all the calculation I have ever seen). This pull request removes the cache in order to save memory.