Remove broken WARNING fallback for unknown sponsor IDs

Markdown inside the wdc-sponsors-page <div> doesn't render because
md_in_html isn't enabled, so the warning showed as literal text. The
fallback was dev-only and any bad sponsor ID is caught during local
mkdocs serve. Addresses PR #118 review feedback.

Author: mrbiggred

docs/sponsors/index.md

@@ -73,8 +73,6 @@ hide:
               </div>
             </div>
           </div>
-        {% else %}
-  > **WARNING:** Unknown sponsor ID `{{ id }}` referenced in `{{ year }}`. Check `data/sponsors.yml` for a typo.
         {% endif %}
       {% endfor %}
     </div>