Remove unused docs.js

What does this MR do and why?

Removes content/assets/javascripts/docs.js. This file no longer does anything on the site, but it's included on every page.

Screenshots, screen recordings, or links to review app

https://sarahg-docs-js.docs.gitlab-review.app/

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.
2. Browse around the site; it should behave and look the same as production. Open the browser console to verify no 404s.

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.