DOC: remove links to jupyter-sphinx-theme

... because it causes a build error and it doesn't seem to be maintained
anymore.

Author: mgeier

doc/usage.ipynb

@@ -786,10 +786,6 @@
     "  [example](https://nbsphinx.readthedocs.io/en/insipid-theme/),\n",
     "  [usage](https://github.com/spatialaudio/nbsphinx/compare/insipid-theme^...insipid-theme)\n",
     "\n",
-    "* [jupyter](https://github.com/jupyter/jupyter-sphinx-theme/):\n",
-    "  [example](https://nbsphinx.readthedocs.io/en/jupyter-theme/),\n",
-    "  [usage](https://github.com/spatialaudio/nbsphinx/compare/jupyter-theme^...jupyter-theme)\n",
-    "\n",
     "* [maisie_sphinx_theme](https://github.com/maisie-dev/maisie-sphinx-theme):\n",
     "  [example](https://nbsphinx.readthedocs.io/en/maisie-theme/),\n",
     "  [usage](https://github.com/spatialaudio/nbsphinx/compare/maisie-theme^...maisie-theme)\n",