CLN: remove CategoricalBlock.to_native_types
The implementation of `to_native_types` on CategoricalBlock seems duplicated with the one of ExtensionBlock (the default for `na_rep` is different, but internally we always pass a value for that and never use the default).
> The implementation of `to_native_types` on CategoricalBlock seems duplicated with the one of ExtensionBlock

is `np.array(values, dtype="object")` equivalent to `values.astype(str)` for Categorical?

> (the default for `na_rep` is different, but internally we always pass a value for that and never use the default)

also could preserve the signature and call super or would we not expect CategoricalBlock to be used by downstream projects?

 > is `np.array(values, dtype="object")` equivalent to `values.astype(str)` for Categorical?

@jorisvandenbossche you can ignore that, I was on an older commit. it's now `np.array(values, dtype="object")` vs `np.asarray(values.astype(object))` > also could preserve the signature and call super or would we not expect CategoricalBlock to be used by downstream projects?

I don't expect anybody to use `to_native_types` (downstream projects use blocks, but typically for construction or accessing the values block-wise). thanks