Replace _has_complex_internals with isinstance(..., ABCMultiIndex)

I've stumbled across this code a few times in groupby and it took me a while to grasp. I'm pretty sure this always evaluates to False, unless we are dealing with a MultiIndex.

For code clarity, I think it would be better if we just did an isinstance check as I don't think the intent of `_has_complex_internals` is very clear