background: fix typo updating images for animated background

We must remove the old image from the cache, not the new one.

This was causing a leak of old (and expensive) background
images, and was causing errors at the end of animations, trying
to destroy the animated background.

js/ui/background.js

@@ -448,7 +448,7 @@ const Background = new Lang.Class({
 
         let image = this._images[index];
         if (image.content)
-            this._cache.removeImageContent(content);
+            this._cache.removeImageContent(image.content);
         image.content = content;
         this._watchCacheFile(filename);
     },