Remove the ability to choose pybinding when installing ExecuTorch

[jathu]

Summary

Fixes #11176

Test plan

CI

rg "\-\-pybind" --hidden -g '!**/{third-party,.git}'

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/11233

• 📄 Preview Python docs built from this PR

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❌ 2 New Failures

As of commit ea9a891 with merge base 9f74777 ():

NEW FAILURES - The following jobs have failed:

• trunk / test-llama-torchao-lowbit / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 134
• trunk / test-qnn-model (fp32, ic3) / linux-job (gh)
  RuntimeError: Command docker exec -t cde201c4f57a156e6f1316d567bd5e335144f150dee41775be71582698d7dd18 /exec failed with exit code 1

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[larryliu0820]

hmm I thought we don't provide the option? If we are providing it, why is this better than --pybind mps

[jathu]

I wanted to leave this option open mainly for development on new backends. After the MPS changes, I think it would make sense to remove this example from public docs