Remove guard duplicating the one in RubyGuards

Author: eregon

src/main/java/org/truffleruby/language/objects/shared/WriteBarrierNode.java

@@ -62,10 +62,6 @@ protected void writeBarrierUncached(DynamicObject value) {
     protected void noWriteBarrier(Object value) {
     }
 
-    protected static boolean isDynamicObject(Object value) {
-        return value instanceof DynamicObject;
-    }
-
     protected boolean isShared(Shape shape) {
         return SharedObjects.isShared(getContext(), shape);
     }