Hi Amir,
Search Results Web Part uses asp.net Panel control to display results (as well as no results message, the panel is the same). Panel translates into div in the markup. The only way to get rid of this div is to develop your own custom search results Web Part.
But if you just need to add a CSS-class for the wrapper somewhere you can use Web Part Containers. It won't be this particular div, but the wrapper on top of it.