Series.droplevel

[ueshin]

Implement Series.droplevel.

https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.Series.droplevel.html

[ueshin]

This might be able to be done at the same time as #1614.

[HyukjinKwon]

@itholic are you interested in this?

[itholic]

@HyukjinKwon Yup, I'm going to take this one.