DEPR: Make dtype="category" always imply ordered=False

[mroeschke]

Currently one can specify dtype="category" to specify data as CategoricalDtype, and since it does not convey ordered information, we treat it as ordered=None internally in order to no-op data that is already categorical. Therefore internally, there are scenarios where we need to be mindful if we have ordered=None vs ordered=True/False

To simplify this gymnastics, it would be great if dtype="category" always meant CategoricalDtype(ordered=False). I think we would only need to issue a deprecation warning when specifying dtype="category" when the data is categorical and ordered=True