Remove JupyterCon 2025 banner from all pages

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](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: jasongrout

_layouts/default.html

@@ -121,30 +121,6 @@
             </ul>
         </div>
     </nav>
-    <header>
-      <div class="jumbotron con-jumbotron">
-        <div class="row">
-            <div class="col-md-12" style='padding:0'>
-                <div class="con-container img-responsive">
-                    <div class="col-lg-8 con-content md-offset-2">
-                        <a href="https://events.linuxfoundation.org/jupytercon/"><img class="con-title img-responsive" src="assets/JupyterCon-Logo-white.svg" alt="The JupyterCon Logo" /></a>
-                        <div class="con-date">November 4-5, 2025</div>
-                        <div class="con-program">
-                            <p>Tutorials Nov 3, Sprints Nov 6</p>
-                        </div>
-                        <div class="con-program">
-                            <p>San Diego, California</p>
-                        </div>
-                        <div class="con-buttons">
-                          <a class="con-button" role="button" href="https://events.linuxfoundation.org/jupytercon/register/" target="_blank">Register</a>
-                          <a class="con-button" role="button" href="https://events.linuxfoundation.org/jupytercon/program/schedule/" target="_blank">View Schedule</a>
-                        </div>
-                    </div>
-                </div>
-            </div>
-        </div>
-    </div>
-</header>
     <section>
       {{ content }}
     </section>