Update the Babel plugin to remove unused top-level constants

[Snuffleupagus]

Looking at the built files there's currently top-level constants that are unused, but still get included in a "disabled" state, see e.g. https://searchfox.org/firefox-main/rev/9d2be0cb8dbf1c3a7ef6ba79be35d637d0327eaa/toolkit/components/pdfjs/content/build/pdf.mjs#93-97

Would it be possible to extend the Babel plugin to remove unused top-level constants?

/cc @nicolo-ribaudo

[nicolo-ribaudo]

Reopening as the PR has been reverted