Remove non-existing empty_like from Sphinx autosummary

[cdeil]

I noticed that empty_like is listed in the top-level function API:
http://xarray.pydata.org/en/latest/api.html#top-level-functions

But I think it doesn't exist in xarray.
This PR removes the entry from the automodsummary list.

Also: I was the 100th person to fork xarray a minute ago.
What's my prize? I could use a better 🚗 or a 🏡 .

[jhamman]

Thanks. This snuck through in #668. FWIW, there is a private method xarray.core.common._full_like.