fix: remove esm->cjs conversion from Webpack plugin

[nmn]

What changed / motivation ?

Remove the Babel plugin that converts ESM to CJS from the Webpack plugin.

[github-actions]

workflow: benchmarks/size

Comparison of minified (terser) and compressed (brotli) size results, measured in bytes. Smaller is better.

@stylexjs/scripts@0.10.1 size:compare
./size-compare.js /tmp/tmp.WNmHadtX1b /tmp/tmp.uKVtDXWhqT

Results	Base	Patch	Ratio
stylex/lib/stylex.js
· compressed	1,013	1,013	1.00
· minified	3,305	3,305	1.00
stylex/lib/StyleXSheet.js
· compressed	1,266	1,266	1.00
· minified	3,776	3,776	1.00
rollup-example/.build/bundle.js
· compressed	567,110	567,110	1.00
· minified	10,232,512	10,232,512	1.00
rollup-example/.build/stylex.css
· compressed	100,628	100,628	1.00
· minified	755,718	755,718	1.00