`params(::MLJType)` method

[ablaom]

I would like to remove this method, as it's essentially "wrong", because it is field-based, instead of property-based. It is my understanding that the method does not appear in any documentation but it may appear in some old tutorials. Unfortunately, it is re-exported by MLJBase, and so removing it will break MLJBase 😢

[ablaom]

In work on MLFlow integration we see that we need such a function. So opening a new issue to add a params2 which in the next breaking release replaces params (but which is not to be exported).