DOC: remove Panel4D from the API docs

[jorisvandenbossche]

Panel4D is deprecated, so I think we can remove it from the API docs.

There is still a small mention of it in the data structures page: http://pandas-docs.github.io/pandas-docs-travis/dsintro.html#panel4d-and-panelnd-deprecated. I think that is sufficient, and still building the API docs (and all its methods) just adds a lot of time to building the docs.

[yinleon]

Hi, I would like to give this a try as my first commit :) Been working on Matplotlib for Docathon.

[jorisvandenbossche]

Closed by #15598