ci(publish): drop publish-gh-pages' cname input

The GitHub Actions Pages deploy that publish-gh-pages uses ignores a CNAME
file, so the input never set the custom domain: Settings → Pages does, and
it already holds python-programming.quantecon.org. From QuantEcon/actions v0.12.0 the action warns on
every deploy that still passes it (QuantEcon/actions#109,
QuantEcon/actions#110).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: mmcky

.github/workflows/publish.yml

@@ -122,7 +122,8 @@ jobs:
         uses: quantecon/actions/publish-gh-pages@v0.11.1
         with:
           build-dir: _build/html
-          cname: python-programming.quantecon.org
+          # Custom domain (python-programming.quantecon.org): set in Settings → Pages. The Actions Pages
+          # deploy ignores a CNAME file, so publish-gh-pages' cname input has no effect.
           create-release-assets: 'true'
           asset-name: 'lecture-python-programming-html'
           github-token: ${{ secrets.GITHUB_TOKEN }}