DOC: remove datareader docs
`io.data` and `io.web` have been deprecated since 0.17 and moved to the `pandas-datareader` project. I am not necessarily saying we should already remove the code (although we could, certainly given that the tests cause failures from time to time, but we can also wait until 0.20), but my proposal is to already remove the docs (and only include a link to the pandas-datareader docs). 

Reason: some of the examples are failing anyways (due to issues with yahoo), our docs already take quite a while to build, and the exact same content is available in the pandas-datareader docs.

Any objections?

@jreback @davidastephens @sinhrks @femtotrader
