Fix VBO for removing a term from content.

#1531 

This PR address https://github.com/openscholar/openscholar/issues/1531#issuecomment-46447009.

With the same scenario mentioned in that comment now a message is displayed:
![selection_257](https://cloud.githubusercontent.com/assets/4497748/3323861/363055ca-f76f-11e3-81ea-ccfe1478d101.png)