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

The complete logic tree functionality has been removed from the engine (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 the SESXMLWriter, consistently with the recent changes in the engine to decouple the gsim logic tree from the source logic tree (see ).