Revert making web_viewer a default feature of rerun_py

[Wumpf]

What

In #6335 web_viewer became a default feature of rerun_py.
This caused a bunch of issues for development:

• pixi run py-build builds with default features and doesn't build new wasm blobs, meaning that the build either fails or picks up outdated
• currently contributor ci doesn't build the web builder during its wheel test job causing failures

Q: Are we sure that wheel publish would still include the web_viewer?
A: Yes. We test this prior to release. This works because we use the pypi feature of build_and_upload_wheels.py. This script also ensures that the wasm blobs are built.

Q: Why does the non-contributor ci build the wasm blobs?
A: It doesn't! Again, we use build_and_upload_wheels.py there which builds with --no-default-features. However, that script can't be used on the contributor ci since that would spill the GCS secrets (upload on that script is not optional right now).

=> Alternative solutions:

• do --no-default-features on pixi run py-build and on the contributor ci (potentially by using and adjusting build_and_upload_wheels.py
• make pixi run py-build slower by making it depend on the web build

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 examples from latest main build: rerun.io/viewer
  • Using full set of examples from nightly build: rerun.io/viewer
• 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
• Recent benchmark results
• Wasm size tracking

To run all checks from main, comment on the PR with @rerun-bot full-check.