Remove obsolete Webpack and Wallaby configuration files

These files are no longer needed after migration to esbuild:
- webpack.config.js (replaced by build.js)
- demo/webpack.config.js (replaced by build-demo.js)
- wallaby.conf.js (referenced Babel + Mocha, now use Vitest)

All builds and tests continue to pass with esbuild + Vitest.

Author: claude