(Re)-deprecate order keyword in factorize

[TomAugspurger]

In #6930 we deprecated order.

As of 0.18.1, the keyword was no-longer documented, and doesn't raise, but wasn't actually removed.

In [3]: pandas.factorize([0, 1], order=True)
Out[3]: (array([0, 1]), array([0, 1]))

In [4]: pandas.__version__
Out[4]: '0.18.1'

[TomAugspurger]

I don't have a strong preference on deprecating vs. just removing. Probably best to deprecate for one release.

[jorisvandenbossche]

To be clear, it seems the order keyword does nothing (so it was forgotten to be removed from the signature).

[EricChea]

I can take this. I'm made some mods to deprecate_kwargs in pandas/util/_decorators.py to accommodate and just need to write some tests.