Fixed CurvePropertyType xlink:href

Fixes reported build failure in GeoServer gs-app-schema-test XlinkGeometryTest when encoding CurvePropertyType with xlink:href:
http://osgeo-org.1560.x6.nabble.com/App-schema-xlinks-and-curves-td5150642.html

The fix is to remove an old getProperty override as the base class has become smart enough to handle this situation.