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.

inc/header.no-header-2.html

@@ -0,0 +1,8 @@
+<!-- START: this line starts content from: inc/header.default.html -->
+
+<div class="header">
+  <img src="%%DOCROOT_SLASH%%png/ioccc.png" alt="IOCCC image by Matt Zucker" width=300 height=110>
+  <h1>%%HEADER_1%%</h1>
+</div>
+
+<!-- END: this line ends content from: inc/header.default.html -->