Removed the SES table

The motivation is to improve the performance for computation with a large number of stochastic event sets.
For instance Chiara has a computation for the risk course with 100,000 SES. The pre-execute phase takes
1405 seconds, where most of the time is spent in writing the 100,000 records in the SES table. By removing
that table the time goes down to 14 seconds (on my laptop).

See https://bugs.launchpad.net/oq-engine/+bug/1300766, tests are running on https://ci.openquake.org/job/zdevel_oq-engine/348