Remove unused download module from rustup-dist

The actual code for handling downloads for rustup-dist is
in `dist`, which in turn hands off to `rustup-utils` and
`download` (the crate). Hopefully this clean-up makes it
a little easier to figure out what's going on with downloads
for future work.

Author: jelford

src/rustup-dist/src/download.rs

@@ -35,6 +35,5 @@ pub mod notifications;
 pub mod prefix;
 pub mod component;
 pub mod manifestation;
-pub mod download;
 pub mod manifest;
 pub mod config;