Removed the obsolete table uiapi.cnode_stats

It is not used by the code. This PR fixes https://bugs.launchpad.net/oq-engine/+bug/1292027.
Notice that if celery is not started the user get the error "No live compute nodes, aborting calculation"
earlier than before. The tests are running: https://ci.openquake.org/job/zdevel_oq-engine/346/