It's a built in plugin function added as a feature by YOOtheme. I believe it's because they have ended support for IE6 in all of their product.
Look in /yoo_master_wp/warp/systems/wordpress/helpers/system.php
Around line 299 you will see a function for outdated browser warnings. I'm not 100% sure, but perhaps simply commenting out that function will resolve the issue.
Don't forget to make a backup first, just in case it doesn't go as expected.
 I might also be horribly, terribly wrong, so that's the best reason to make a backup first... Haha! :-)