drop extra::par

This was a dead end experiment, and not a sensible way of implementing
generic data parallelism. This also removes the `graph500-bfs.rs`
benchmark because it relies on `extra::par`.

Closes #5626

Author: thestinger

src/libextra/extra.rs

@@ -89,7 +89,6 @@ pub mod glob;
 pub mod term;
 pub mod time;
 pub mod arena;
-pub mod par;
 pub mod base64;
 pub mod rl;
 pub mod workcache;