Removes superClass field from JReferenceType in favor of pushing down to some subclasses.

This fixes duplication between JReferenceType and JDeclaredType.

Suggested by: grek
Review by: spoon


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@8581 8db76d5a-ed1c-0410-87a9-c151d255dfc7