DOC: Removed duplicate doc line
This line of documentation is already included in the parent shared docs. So the line appears twice. See the screenshot below from [the Dataframe.aggregate page](https://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.aggregate.html). My patch cleans that up.

![screenshot from 2019-01-07 17-00-12](https://user-images.githubusercontent.com/9993/50803035-ca04fa00-129d-11e9-8078-a0a3d9a55400.png)
