Skip unneeded search bundles when compiling JavaScript

What does this MR do and why?

The site only runs one search backend at once, as determined by the SEARCH_BACKEND environment variable. By dropping files related to the unused search backend, we can reduce compile time on CI by a few minutes.

Closes #1597 (closed)

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. Check out this branch
3. Compile the site with Lunr: SEARCH_BACKEND="lunr" make compile
4. In the Yarn output, right after INFO: Compiling JavaScript..., you should see lunrsearch.js, but not google.js, google_results.js, or google_search_api.js
5. View the site and verify Lunr search works as expected: bundle exec nanoc view
6. Compile the site with Google: make compile
7. Check the Yarn output and verify the google*.js files are processed, but not lunrsearch.js
8. View the site and verify Google search works as expected: bundle exec nanoc view

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.

Closes #1597 (closed)