remove Hibernate specific annotation

Remove Hibernate specific annotation:
https://github.com/aerogear/aerogear-unifiedpush-server/blob/master/src/main/java/org/jboss/aerogear/unifiedpush/jpa/PersistentObject.java#L32-L36

This causes issues on other JPA implementations, like OpenJPA

Note: the UUID is not really covered in the JPA standard... hence we are currently relying on that Hibernate specific API