Remove Objectron from default examples

[emilk]

What

The first-frame heuristics of objectron sometimes produce really bad results:

(top-right corner is a 3D view with 2D elements in it).

In the interest of getting a release out today, and not giving first users a bad impression, this PR removes Objectron from the example page. It is not a very pretty example anyway.

Related issues

• Investigate first-frame heuristics for our default examples #5156
• Run all examples nightly #5154

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!

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

[abey79]

🤷🏻