fix emloty 2nd level header warning

Removed empty 2nd level header from top level `index.html`
to remove Nu HTML check warning.

Run `make gen_top_html` to perform the above and test under macOS.

index.html

@@ -46,7 +46,6 @@
 <div class="header">
   <img src="./png/ioccc.png" alt="IOCCC image by Matt Zucker" width=300 height=110>
   <h1>The International Obfuscated C Code Contest</h1>
-  <h2></h2>
 </div>
 
 <!-- END: this line ends content from: inc/header.default.html -->