Removed task_no

Having fixed the ordering problems in https://github.com/gem/oq-engine/pull/1506, now it it possible to remove the `task_no` and the useless `order_by task_no`. I am removing it from the code, in the future it could be removed from the database. Tests running here: https://ci.openquake.org/job/zdevel_oq-engine/613
Please specify the meaning of number 242