Remove dark-theme hack on next Xarray release

[kmpaul]

What is your issue?

With the merging of https://github.com/pydata/xarray/pull/6501 and the closing of https://github.com/pydata/xarray/pull/6500, the hack for Xarray HTML reprs in the Furo Sphinx theme (implemented in #72) should no longer be needed. I don't believe that it will cause problems if it remains, but if it is no longer needed, I believe it should be removed.

The easiest way of satisfying this need is to simply remove the docs/_static/custom.js file and references to it in docs/conf.py.