Remove RiskCalculation object

This concludes a refactoring work started several months ago (see https://bugs.launchpad.net/oq-engine/+bug/1374298).
Now not only there is no RiskCalculation object in the database, but also the RiskCalculation class has been removed and the configuration parameters both for hazard and risk are stored in the object oqparam.
The tests are running here: https://ci.openquake.org/job/zdevel_oq-engine/920/