Remove Model

[greenape]

We should, I think, remove Model and ModelResult from flowmachine.

They're a neat trick, but in practice have not been used. We would be better off dealing with 'I want to run a python thing and put it in the db' through a more considered route (e.g. just run the damn python in the db). They also introduce a bunch of 'clever' code that is a maintenance headache.

Would also make unpicking #5140 a little easier.