Remove std::io::signal

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]

src/libstd/io/mod.rs

@@ -21,7 +21,7 @@
 `std::io` provides Rust's basic I/O types,
 for reading and writing to files, TCP, UDP,
 and other types of sockets and pipes,
-manipulating the file system, spawning processes and signal handling.
+manipulating the file system, spawning processes.
 
 # Examples
 
@@ -275,7 +275,6 @@ pub mod fs;
 pub mod net;
 pub mod pipe;
 pub mod process;
-pub mod signal;
 pub mod stdio;
 pub mod test;
 pub mod timer;