Docs still available at URL with old crate names

[kmcallister]

Somehow I ended up at http://static.rust-lang.org/doc/std/dbg.html and was confused because this module is now extra::dbg, but the old URL still works. Probably this is a stale file that should be removed.

[thestinger]

Closing in favour of #7476