Remove builds and usage of demo.rerun.io

[jprochazk]

What

Part of #4277

There's still usage of the build_demo_app script in the screenshot comparison tool, and I wasn't feeling quite brave enough to also rip it out from there in this PR.

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested the web demo (if applicable):
  • Full build: app.rerun.io
  • Partial build: app.rerun.io - Useful for quick testing when changes do not affect examples in any way
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG

• PR Build Summary
• Docs preview
• Examples preview
• Recent benchmark results
• Wasm size tracking

[jprochazk]

The build_demo_app.py script is left untouched, because it was also used by the screenshot comparison tool. Instead of patching it and trying to make it work, I rewrote it in Rust. I'm more convinced than ever that we should actively push to rewrite all of our scripts as soon as possible, just because of the staggering difference in DX between writing this kind of code in Rust vs Python

[emilk]

Please write an issue for finishing up the port of build_demo_app.py, and then work on that next. Porting to Rust is good, but duplicating scripts is not :)

[jprochazk]

• Remove build_demo_app.py #4428