Removed unused images and style (Related to Issue 271)

Based on this issue https://github.com/NuGet/NuGetGallery/issues/271 I found 2 images that are not used in the entire solution. I also discovered that the logo is styled twice, but the style in the site.css points to a incorrect logo and is overwritten by the layout.css, so I removed this as well.

All other images seems to be used at least in the css - but I didn't check if the style is applied somewhere..