tests: Remove download-puppeteer-browser script
Created by: chrismwendt
It was added in https://github.com/sourcegraph/sourcegraph/pull/19105 to fix Puppeteer failing to download for an unknown cause.
We've since upgraded to Puppeteer 12 #28783, so it seemed worth trying again.
This doesn't increase CI build time:
- Before 11m11s https://buildkite.com/sourcegraph/sourcegraph/builds/121858
- After 11m17s https://buildkite.com/sourcegraph/sourcegraph/builds/121908
cc @marekweb