Remove setup_web script - pixi handles everything except wasm anyways already

[Wumpf]

What

• Part of Replace our setup*.sh files with pixi #4170

pixi already handles binaryen. the rust target is ok to throw in as an instruction since it's fairly obvious

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):
  • Using newly built examples: app.rerun.io
  • Using examples from latest main build: app.rerun.io
  • Using full set of examples from nightly build: app.rerun.io
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG
• If applicable, add a new check to the release checklist!

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