Remove unused jQuery setup hooks

[adamtheturtle]

Summary

• remove the unused setup_jquery functions from the documentation and demo configurations
• remove the now-unused Sphinx imports

These functions were never registered as Sphinx event handlers or called directly, so they could not inject jQuery. Removing the dead hooks eliminates the unnecessary reflective access to html_js_files without replacing it with another typing workaround.

[jdillard]

Nice find. I agree that both hooks are unregistered and unreferenced, so removing them and their imports preserves existing behavior.