Remove sphinx_rtd_theme from doc

I realized that I inlined the sphinx_rtd_theme into the doc directory when I set up the versioned documentation build, but this is no longer needed with the way that other CESM components are doing these versioned doc builds. I don't want to get side-tracked into this right now (partly because there may be some other changes needed), but one of us should remove this (doc/source/_themes/sphinx_rtd_theme) and fix the documentation build to use an installed version of this theme, as we do for CESM, CIME and CTSM. I can help with this.