Removing interpreter should also remove credential associated with it.

When an interpreter is removed, the credentials associated with this interpreter is still in the environment. It would be better to remove this automatically when the interpreter is removed so that it reduces another step that user needs to remember to do.

Steps:
1. Create an interpreter - call jdbc2 / w/ jdbc group
2. Navigate to credentials page, and create an credential associated with it.
3. Navigate back to interpreter page, and delete that interpreter.
4. Navigate to credentials page. The credential associate with the interpreter still exists.

Desired out come is that this credential is also removed.