Removed the rest of the stuff related to the supervisor

The postgres user oq_job_superv has been removed. While I was at that, I also removed the user oq_reslt_writer,
thus reducing by half the number of connections to the database. This is solving at once two long standing issues:
https://bugs.launchpad.net/oq-engine/+bug/1248803 and https://bugs.launchpad.net/oq-engine/+bug/1248801
For a clean diff, https://github.com/gem/oq-engine/pull/1321 must enter first.