Clear record when doc_type was changed

@czue http://manage.dimagi.com/default.asp?140716#785508
the `change_transform` function is not intended to be the place where any actual work happens. it would be more appropriate to have change_transform return something like: 
```
{
    'doc_dict': doc_dict,
    'deleted': True
}
```

and then have `change_transport` check for that format and delete the document.

it would also be great if this deleted fluff documents from couch as well, but if you don't want to support that please just make a comment in the code so it's clear that it doesn't work yet.