I believe that you are using free version of this editor: http://www.q2apro.com/plugins/sceditor
He is adding a google adsense ad to the site for the free version, you can buy the paid version to remove that...
It is in this folder and file: /q2apro-sceditor-free/q2apro-sceditor-layer.php
Plugin owner note:
// NOTE: You are NOT permitted to change the following code, if you do so, you break the granted license and can be sued legally.
// If you do not like ads to be displayed, you have to buy the ads-free premium plugin at: http://www.q2apro.com/plugins/sceditor