datanucleus (as in org.datanucleus...) is the JDO/JPA library, so that message indicates that you still have JDO/JPA artificacts in your project. The message is correct - now that you are using objectify your class is not persistable using datanucleus, so what is still trying?
I would suggest that you review your project settings and your class file's import statements to make sure you've removed all JDO/JPA.