Decouple the gsim logic tree from the SES output and remove dead code

The complete logic tree functionality has been removed from the engine few days ago (see https://bugs.launchpad.net/oq-engine/+bug/1189397) so 200+ lines can be removed from nrmllib. This PR also removes gsim_lt_path from the signature of SESXMLWriter, consistently with the recent changes in the engine to decouple the gsim logic tree from the source logic tree (see https://bugs.launchpad.net/oq-engine/+bug/1248237).

The tests are green: http://ci.openquake.org/job/zdevel_oq-nrmllib/11/