Summary

Remove the in-container Envoy proxy from both public browser images:

• Delete the package installer, supervisor service, xDS bootstrap/init scripts, and baked localhost proxy certificate/trust setup.
• Remove wrapper startup/readiness handling and snapshot seed-proxy cleanup, along with their now-unused helpers.
• Remove the xDS-specific known environment key while preserving generic fork identity, instance JWT, metro identity, telemetry, and API configuration.
• Document external-proxy configuration and add headful/headless regression coverage for direct and external-proxy navigation with legacy identity variables present.

Compatibility / deployment gate

Breaking change: do not deploy these images while any allocation, fallback, or snapshot configuration still passes --proxy-server=https://127.0.0.1:3128. That listener no longer exists. Update those configurations to use an external proxy (or direct networking) first. Legacy XDS_SERVER configuration no longer starts a local proxy.

Generic Chromium proxy flags and proxy environment variables are unchanged. This PR does not change deployment routing or remove the instance credentials used by other services.

Validation

• Built both Docker images successfully.
• Passed TestBrowserNetworkingWithoutLocalProxy: headful/headless × direct/external HTTP proxy, including absence of the Envoy binary, config, and certificates.
• Passed separate headful/headless × root/non-root smoke tests for snapshot preparation, fork-identity handoff, Chromium restart, and HTTPS navigation.
• Passed non-e2e race tests and vet with cmd/shell excluded. An existing Chromium temporary-directory cleanup failure in lib/devtoolsproxy passed on rerun; the subsequent complete non-e2e run passed.
• Full tests/vet remain blocked by the pre-existing cmd/shell/main.go:160 UUID/string argument mismatch, reproduced on unmodified main (d260d1b).
• Full e2e, platform deployment, snapshot restore, and WebRTC playback were not run.