CLN: Revert internal CategoricalDtype.ordered -> CategoricalDtype._ordered references 
xref https://github.com/pandas-dev/pandas/pull/26403#discussion_r299144624

This will need to be done as part of the process for changing the default value of `CategoricalDtype.ordered` from `None` to `False`, which should be part of the 1.0 release.  Opening this issue to serve as a reminder.
