[docs] Fix theme toggle not displaying due to duplicate const declara… (#2092)

Minor fix. Related to PR
##2090

**Link the Issue(s) this Pull Request is related to.**
- #1894

**Summarize your change.**
Remove duplicate `const auxNavList` line that was accidentally
introduced in commit fa916ff, causing a JavaScript syntax error that
prevented the theme toggle button from rendering.

Author: ramonfigueiredo

docs/_includes/header_custom.html

@@ -2476,7 +2476,6 @@
       });
 
 
-      const auxNavList = document.querySelector('.aux-nav-list');
       const auxNavList = document.querySelector('.aux-nav-list');
       if (!auxNavList) {
         console.warn('aux-nav-list not found, cannot add theme toggle');