Remove the mock model (reviewed, pending merge)

We had discussed doing this in the past.  We are no longer really maintaining the mock model.  None of our downstream dependencies ever used it, and a mimic plugin is probably the correct way to go to give people a way to hit a fake otter service.

Keeping the mock just means that every time we change something, we have to update the mock tests.  I think that so long as we maintain the interface/abstraction between the store and the rest of otter, we're ok.