Remove unnecessary configuration of tabindex (#11813)

This was called out by the lighthouse accessibility report.

Author: parlough

src/_includes/header.html

@@ -1,7 +1,5 @@
-{% assign route = page.url|regex_replace:'/index$|/index\.html$|\.html$|/$' %}
-
 <header class="site-header">
-  <a href="#document-title" id="skip-to-main" class="filled-button" tabindex="1">
+  <a href="#document-title" id="skip-to-main" class="filled-button">
     Skip to main content
   </a>
   <nav class="navbar">