Removed calc_num task properly

PR https://github.com/gem/oq-engine/pull/1306 removed the column num_tasks from the job_stats table, since it was redundant. It also removed a call to calc_num_tasks that (as a side effect) was determining the preferred_block_size in the event based calculator. That broke qa_tests/hazard/event_based/case_12.So it seems that there is still a hidden dependency from the block size that requires further investigation. This PR removes num_tasks properly and fixes the test.