Method cleanup

src/api/java/Z3ReferenceQueue.java:114–119

(Context ctx)

        }

        private void cleanup(Context ctx) {
            decRef(ctx, nativePtr);
            assert (prev != null && next != null);
            prev.next = next;
            next.prev = prev;
        }

        private void insert(Reference<?> ref) {
            assert next != null;