Remove namespace prefix from assetRef attribute.
Remove namespace prefix from assetRef attribute.

This is consistent with (my understanding of) the schema definition
and with the other NRML attributes defined in xml.py.


Former-commit-id: d32f8c7a08f7bf440f51316b30c460022d99a72f [formerly 6a1757f4513a1d3fe365d57648f652418a85575e]
Former-commit-id: 29656e273b083d9eb8c75a0a5dc14b76af21e475
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine