CLN: private impl of OrderedDefaultDict can be removed

[jreback]

https://github.com/pandas-dev/pandas/blob/master/pandas/compat/__init__.py#L376

I think this was leftover from 2.6 compat.

[rosygupta]

@jreback So the private methods need to be made public? Is that all is required?

[jreback]

no i think it can just be removed

[TomAugspurger]

And replaced with an import from collections import OrderedDict.