Add extension removal methods

[jisantuc]

With the FooExtension typeclasses it's very easy to add extensions. However sometimes it would be useful to be able efficiently to remove extensions. For example, in Franklin, items are validated on serialization on all of the endpoints off of collections. As a result the returned items are never going to be equal to the items a user posts. It would be nice to have a one-liner to remove the validation extension fields. Another possibility is that a user has created a halfway invalid label item and rather than try to edit a few things would rather just delete the existing extension info and start over. This shouldn't be that difficult and I think could substantially improve downstream tests.