The faces cannot and should not be removed. They are the diffuse color of a material. It's probably a jpg image. That material should also have a clip or alpha map. Which can also be a jpg image. That clip/alpha is black and white image where black goes for the empty space and white the leaf itself. What you can do is create a png image based on the diffuse+alpha maps in an image editor. Use the diffuse jpg as base image and insert the clip/alpha as mask. Then save it as png. Go to Sketchup material editor and replace the texture by this png.