Remove obsolete codex-cli README (#17096)

Problem: codex-cli/README.md is obsolete and confusing to keep around.

Solution: Delete codex-cli/README.md so the stale README is no longer
present in the repository.

.github/workflows/ci.yml

@@ -63,10 +63,5 @@ jobs:
       - name: Check root README ToC
         run: python3 scripts/readme_toc.py README.md
 
-      - name: Ensure codex-cli/README.md contains only ASCII and certain Unicode code points
-        run: ./scripts/asciicheck.py codex-cli/README.md
-      - name: Check codex-cli/README ToC
-        run: python3 scripts/readme_toc.py codex-cli/README.md
-
       - name: Prettier (run `pnpm run format:fix` to fix)
         run: pnpm run format