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.