Remove useless import statements

On generated classes by sourceGen using the 'type' method, some (apparently)
useless import statements are included since almost every used class is fully
qualified.

The useless imports are:
+ on XYZ.java:

• import java.io.IOException;
• import java.io.Reader;
• import java.io.Serializable;
• import java.io.Writer;
• import java.util.Date;
• import org.exolab.castor.xml.MarshalException;
• import org.exolab.castor.xml.ValidationException;
• import org.xml.sax.ContentHandler;
  + on XYZDescriptor.java:
• import org.exolab.castor.mapping.AccessMode;
• import org.exolab.castor.xml.TypeValidator;
• import org.exolab.castor.xml.XMLFieldDescriptor;

Castor-XML could either avoid fully qualifying the classes used on the generated
source (may it's not possible if it's using reflection compulsively but would
certainly improve readibility of reverse engineered UML diagrams...), or stop
declaring the useless imports.

Anyway, it would be very nice to get rid of the warning messages of the Eclipse IDE.