Fix vbo actions for removing/apllying terms to content.

#1531 

Addressing https://github.com/openscholar/openscholar/issues/1531#issuecomment-46325993.

When removing a term from a node that is not associated with that vocabulary we get this message:
![selection_256](https://cloud.githubusercontent.com/assets/4497748/3309999/2a0145bc-f6ab-11e3-8539-2148a406f4a5.png)