Fix broken benchmarks build (or drop React)

[mourner]

After merging publisher-production branch with the docs upgrades, yarn run build-benchmarks started failing, with rollup-plugin-commonjs complaining about unexpected token in react-dom (although the exact location is bogus — there's nothing suspicious there). This package upgraded from v16.0.0 to v16.7.0, and something in between those versions broke the setup. I tracked it down specifically to v16.4.0, starting from which the builds fail. Also confimed that the builds still fail in v16.8.2.

This is really cryptic and I don't have yet good ideas about what causes this, but I think the best fix would be to refactor and cut out all this React bloat from benchmarks for good. 😡

[mourner]

I think I found the likely culprit of the original issue — it's rollup-plugin-unassert breaking on some code because of an outdated Acorn dependency — the plugin hasn't been updated in a while. I'll see if I can get maintenance access to the repo and make a new release.