Fix the failure of "Build docs" workflow (ipython#14544)

Fixes ipython#14542.

Local run of the following command (along with previous steps) failed
before deleting this line, and succeeded afterwards.

```
make -C docs/ html SPHINXOPTS="-W" \
          PYTHON="coverage run -a" \
          SPHINXBUILD="coverage run -a -m sphinx.cmd.build"
```

Author: Carreau

docs/source/conf.py

@@ -131,7 +131,6 @@
 
 # Options for HTML output
 # -----------------------
-html_theme_path = [sphinx_rtd_theme.get_html_theme_path()]
 # - html_theme
 # - html_static_path
 #     Add any paths that contain custom static files (such as style sheets) here,