`serde-deny-unknown-fields` pointless in `model`

[tcharding]

The deny unknown fields attribute was added so that in testing we catch mistakes in what Core returns and what we expect to get. In model we derive serde traits so that users of the lib can persist the types if they want to, the model types are never used directly with Core. It does not make sense to have the deny unknown fields attribute in model.

100% my mistake for not noticing this during review when we added the attribute, and then again later when we feature gated the attribute.

[jamillambert]

Definitely my fault. When I added it I just search and replaced to add it below all occurrences of the derive line above, but it doesn't do anything in model, and there are hundreds of structs that all look similar, so went unnoticed. Easy fix at least (#374).