Cull unused complex types

[duck2]

Currently, if a schema like

  <xsd:complexType name="person">
    <xsd:sequence>
      <xsd:element name="name" type="xsd:string"/>
      <xsd:element name="born" type="xsd:string"/>
      <xsd:element name="died" type="xsd:string" minOccurs="0"/>
    </xsd:sequence>
  </xsd:complexType>


  <xsd:complexType name="author">
    <xsd:complexContent>
      <xsd:extension base="person">
        <xsd:attribute name="recommends" type="xsd:IDREF"/> <!-- Book -->
      </xsd:extension>
    </xsd:complexContent>
  </xsd:complexType>

is given and only author is referred from the root element, we still generate code for both person and author. This is unnecessary.