Don't include sample images in update packages

This modifies the 3.x build script to exclude the sample images we distribute from the update packages.  They serve no real purpose once a site is established and site owners who wish to remove this content should be free to do so.  Note that the full install package is unaffected and these images will continue to be bundled there.

I've done this as a PR to solicit feedback in case there are individuals who may think these are important to retain in the update packages.