DOC: skipna not an argument of drop_duplicates
From what I can tell, `skipna` keyword was both added and subsequently
removed in #1244. However, the docstring for DataFrame.drop_duplicates
still lists it as a parameter.

I remove this from the docs, so they accurately represent the code.
An alternative to this would be to put back the skipna functionality.

pinging @changhiskhan, since he was the one that worked on this.
