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/reflect/Field.java

@@ -254,10 +254,6 @@ public class Field<T> extends AccessibleObject {
     return getAnnotations();
   }
 
-  public boolean isEnumConstant() {
-    throw new UnsupportedOperationException();
-  }
-
   private static native long getPrimitive
     (Object instance, int code, int offset);