CLN: Revert internal CategoricalDtype.ordered -> CategoricalDtype._ordered references

[jschendel]

xref #26403 (comment)

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.

[TomAugspurger]

@jschendel are you likely to have time for this for 1.0?

[jschendel]

Yes, I can do this for 1.0