Remove old language about `use`
Remove old language about `use`

We haven't been using `use` forever, but we still should explain it.
dskecse/rust_for_rubyists,steveklabnik/rust_for_rubyists,gpbonillas/rust_for_rubyists,steveklabnik/rust_for_rubyists,dskecse/rust_for_rubyists,steveklabnik/rust_for_rubyists,gpbonillas/rust_for_rubyists,dskecse/rust_for_rubyists,gpbonillas/rust_for_rubyists