Removed the dependency from nrmllib

See https://bugs.launchpad.net/oq-engine/+bug/1379827. The tests are green: https://ci.openquake.org/job/zdevel_oq-engine/736
There actually a docstring which is clear enough:
```
        Serialize a list of values produced by iterating over an instance of
        :class:`openquake.commonlib.risk_parsers.ExposureParser`
```