(#855): Don't cargo-vendor the Rust dependencies anymore
Librsvg's release tarballs started including the "cargo vendor" sources for its Rust dependencies when Rust got first introduced into the project, to make it as easly as possible for distributions to keep building tarballs as usual.
Nowadays, most distros have tooling to pre-download Rust dependencies for packages that use Rust. So, let's stop vendoring the deps into the tarballs, to make them much smaller.
Fixes #855 (closed).
Closes #855 (closed)