Remove JupyterCon 2025 announcement banner (#7757)

This removes the JupyterCon 2025 promotional announcement banner
from the documentation configuration.

Reverts announcement from PR #7727
See jupyter-governance/ec-team-compass#146

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <[email protected]>

Author: jasongrout

docs/source/conf.py

@@ -172,7 +172,6 @@
 # further.  For a list of options available for each theme, see the
 # documentation.
 html_theme_options = {
-    "announcement": '🚀 Join us in San Diego · JupyterCon 2025 · Nov 4-5 · <a href="https://events.linuxfoundation.org/jupytercon/program/schedule/">SCHEDULE</a> · <a href="https://events.linuxfoundation.org/jupytercon/register/">REGISTER NOW</a>',
     "header_links_before_dropdown": 5,
     "icon_links": [
         {