Remove unit

Closes https://github.com/rust-lang/rust/issues/18614.
This ends up being a parse error so it doesn't matter too much what AST node this parses to.

I imagine we'll want the obsolete syntax knowledge to stick around for quite some time. There is still a lot of outdated Rust code in Stack Overflow pages that it would be nice to have the compiler be helpful with. Though I imagine most of those examples don't parse anyway for other reasons...