Don't export `UnivariateFinite`.

[ablaom]

The fact that this is exported is necessitating a hack in MLJBase/src/MLJBase.jl to enable UnivariateFinite code to be replaced with CategoricalDistributions.jl as a dependency.

And remove the doc-string as a duplication of one now provided at CategoricalDistributions?

[OkonSamuel]

@ablaom I won't suggest removing the doc-string though, since developers relying on the lightweight MLJModelInterface may not also want to depend on the heavier MLJBase/CategoricalDistributions. They may still want to query the docstring for UnivariateFinite object.