Good question.
It is a long time since I set this up on my site.
Since my site search results also have borders round them I'm afraid I cannot answer this question quickly..
Here is a link which looks like it may have the answer, but if Gogole don;t want you to do it - maybe it is best not to do it??
Removing Borders from Ad Sense Search Results - AdSense Help
PS - in my experience so far - it is not worth spending too much time on your search as the earnings are fairly small compared to other adsense units.