remove GNU Classpath and Apache Harmony compatibility code

Rather than try to support mixing Avian's core classes with those of
an external class library -- which necessitates adding a lot of stub
methods which throw UnsupportedOperationExceptions, among other
comprimises -- we're looking to support such external class libraries
in their unmodified forms.  The latter strategy has already proven
successful with OpenJDK's class library.  Thus, this commit removes
the stub methods, etc., which not only cleans up the code but avoids
misleading application developers as to what classes and methods
Avian's built-in class library supports.

classpath/java/lang/ThreadGroup.java

@@ -13,7 +13,7 @@ package java.lang;
 import avian.Cell;
 
 public class ThreadGroup implements Thread.UncaughtExceptionHandler {
-  final ThreadGroup parent; // package private for GNU Classpath compatibility
+  private final ThreadGroup parent;
   private final String name;
   private Cell<ThreadGroup> subgroups;