XmlSerializer: Empty <xsd:import /> tag in generated WSDL

[vidrenning]

This is an issue that got introduced by my commit for #1014. Sorry about that!

In that commit I wanted to remove the line <xsd:import namespace="http://schemas.datacontract.org/2004/07/System" /> from the generated WSDL. However in the actual commit, I only removed the namespace="http://schemas.datacontract.org/2004/07/System" part, so an empty <xsd:import /> tag remains in the WSDL.

It turns out that this throws off some third party SOAP parsers, so it is better to remove it.