Remove site_collection column from the database

The goal is to save memory and remove duplication: the site_collection pickled field is not needed nowadays, since the site_collection can be extracted from the site table. This pull request is enough to solve the memory problem for the South America computation described in https://bugs.launchpad.net/oq-engine/+bug/1193223.
For a clean diff, https://github.com/gem/oq-engine/pull/1219 should enter first.

NB: the interface did not change and all tests pass, see http://ci.openquake.org/job/z_oq-engine_template/63/