The `std::io::signal` API was only implemented under `librustuv`, which
is now being removed. Rather than keep around an unimplemented API, this
commit removes it altogether.
See the [runtime removal
RFC](https://github.com/rust-lang/rfcs/pull/230) for more context.
See [green-rs](https://github.com/alexcrichton/green-rs/) for a possible
migration path for signal handling code, although in the long run we
plan to add native signal handling to `std::io`.
[breaking-change]