Remove sufia-models as a dependency

[flyingzumwalt]

After #41

• [ ] Move all necessary code (and associated tests) from sufia-models into curation_concerns-models and/or hydra-works
• [ ] Remove sufia-models from the gem's dependencies

Update code and tests accordingly.

See Code Shredding doc for a guide

[kevinreiss]

The sufia-models to curation_concerns-models is underway in https://github.com/pulibrary/hydra-curation_concerns/tree/sufia_move