Drop risk calculation table

See https://bugs.launchpad.net/oq-engine/+bug/1374298. Notice that a companion in commonlib is required: https://github.com/gem/oq-commonlib/pull/47
The tests are running here: https://ci.openquake.org/job/zdevel_oq-engine/779
This is correct, but I can add a comment. The table oq_job has a field hazard_calculation_id which is NULL if the job is of kind hazard and NOT NULL if the job is of kind risk (in that case it contains the hazard job that preceded the current risk job).