Remove duplicate background switcher functionality

## 🧹 Cleanup Changes
- Removed duplicate background-switcher.js file
- Updated head.html to remove background-switcher script reference
- Cleaned up redundant background theme selection feature

## 🎯 Background Features Status
The blog still maintains its core background functionality through:
- Dark mode toggle (retained)
- Responsive design backgrounds (retained)
- CSS gradient backgrounds (retained)

Removed redundant theme selector with:
❌ 海洋 (Ocean)
❌ 日落 (Sunset)
❌ 森林 (Forest)
❌ 星空 (Galaxy)
❌ 极光 (Aurora)
❌ 默认 (Default)

This eliminates duplicate UI elements and improves user experience.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: calderbuild

_includes/head.html

@@ -332,9 +332,6 @@
     <script data-ad-client="ca-pub-6487568398225121" async
         src="https://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js"></script>
 
-    <!-- Background Switcher -->
-    <script src="{{ "/js/background-switcher.js" | prepend: site.baseurl }}"></script>
-    
     <!-- Dark Mode Toggle -->
     <script src="{{ "/js/dark-mode.js" | prepend: site.baseurl }}"></script>