Remove unused fields from models

[DeD1rk]

What

Some models may include fields that are currently returned by the API, but that we don't use. Those should be removed, so that the app keeps working even if the fields are removed from the API.

Why

As of svthalia/concrexit#2796 (and probably some other similar changes to decouple concrexit's apps), events will no longer have a slide field. IIRC reaxit tries to parse that field but doesn't use it. If we don't change reaxit not to try parsing such fields, it might break with future versions of concrexit.

How

Check for any fields that don't seem useful, check that they're not used, and remove them. Then run build_runner again to update the model generated code.

[DeD1rk]

I just checked that slides aren't parsed so that's no problem. So there might not actually be anything that really needs to be changed.