Hazard post-processing code cleanup

This removes a bunch of obsolete code and tests.

I'm doing this in preparation for https://bugs.launchpad.net/openquake/+bug/1082312, which will add a UHS post-processing feature. This feature will be sort of the next step in the calculation/post-processing pipeline:

curves -> mean/quantile curves -> maps (from all curves) -> UHS (from all maps)