Remove JupyterCon 2025 banner from all pages

[jasongrout]

This removes the JupyterCon 2025 promotional banner from the default layout, which was displaying across all pages of jupyter.org.

Reverts PR #821
See jupyter-governance/ec-team-compass#146

🤖 Generated with Claude Code, reviewed by @jasongrout

[jasongrout]

link check is failing for some of the recently refactored governance pages. I think those should be fixed on a separate issue.

_ /home/runner/work/jupyter.github.io/jupyter.github.io/_site/community.html: https://github.com/jupyter/governance/blob/main/conduct/code_of_conduct.md _
https://github.com/jupyter/governance/blob/main/conduct/code_of_conduct.md: 404: Not Found
_ /home/runner/work/jupyter.github.io/jupyter.github.io/_site/media_submissions.html: https://jupyter.org/governance/charters/MediaStrategyCharter.html#editorial-principles _
https://jupyter.org/governance/charters/MediaStrategyCharter.html#editorial-principles: 404: Not Found
_ /home/runner/work/jupyter.github.io/jupyter.github.io/_site/social.html: https://jupyter.org/governance/charters/MediaStrategyCharter.html _
https://jupyter.org/governance/charters/MediaStrategyCharter.html: 404: Not Found

[jasongrout]

@choldgraf - it seems like https://jupyter.org/governance/charters/MediaStrategyCharter.html should be one of the redirects? Did we miss this one?