Revert "Add Auth0 client_credentials auth to simulation gateway calls"

[MaxGhenis]

Summary

Reverts #3471. Master GAE deploy is broken because the four org-wide GATEWAY_AUTH_* GitHub Actions secrets Anthony set on the PolicyEngine org are not visible to this repo. The secrets resolve to empty strings in push.yml, gcp/export.py substitutes empty strings into the Dockerfile, and GAE Cloud Build errors out with:

Error response from daemon: failed to parse Dockerfile: ENV must have two arguments

Failed deploy run: https://github.com/PolicyEngine/policyengine-api/actions/runs/24595001267

Impact

• Prod GAE has not been updated; still running pre-3471 code (unauthenticated calls to sim gateway, which is currently fine because prod sim gateway is still pre-Update PolicyEngine US to 0.324.0 #458 with no auth requirement).
• Every subsequent "Update PolicyEngine API" version-bump push will fail deploy until this is reverted.

Next steps

1. Merge this revert to unstick master deploys.
2. Anthony: either (a) make the four GATEWAY_AUTH_* org-wide secrets accessible to policyengine-api (currently visible only to policyengine-api-v2), or (b) set them as repo-scoped secrets on policyengine-api directly.
3. Reopen the auth-client PR (or cherry-pick 3471 onto a new branch) and merge after confirming the deploy env injects real values.

🤖 Generated with Claude Code

[anth-volk]

Apologies, mistakenly set the secrets for "private repositories" instead of "all repositories," so the secrets were not available. I have updated. Please try again.