Since we're shipping prebuilt binaries for all supported platforms, there's no reason to include the src, vendor directories nor any build scripts / cmake configurations.

This was mentioned as an a-aside in a comment on another PR https://github.com/realm/realm-js/pull/3626#issuecomment-825608686

I wouldn't consider this a breaking change, since all public documentation I can find instructs users to git clone our repository to build from source.