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
auto merge of #9380 : thestinger/rust/par, r=alexcrichton

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