Removes factories

[JasonTheAdams]

This PR removes factories as a feature from the library. There are a few reasons for this:

1. It's not really a feature that's necessary for models to do their things; it's a nice-to-have
2. Folks are pretty confused as to when factories are meant to be used, which results in less ideal practices
3. To really work as intended it should use something like Faker, which is a massive library that we don't want as a dependency here
4. It's unclear how this fits into the interfaces and makes for awkward usage

I chatted with @dpanta94 and @jonwaldstein, and we agreed that, while it's a cool feature, it would be better to invert it and have separate library that references models for seeding, rather than have models reference factories.