Remove signal handling.

Since signal handling was only implemented through librustuv, which is
now gone, there's no reason to even provide the API.

[breaking-change]

src/libnative/io/mod.rs

@@ -301,8 +301,4 @@ fn tty_open(&mut self, fd: c_int, _readable: bool)
             })
         }
     }
-    fn signal(&mut self, _signal: int, _cb: Box<rtio::Callback>)
-              -> IoResult<Box<rtio::RtioSignal + Send>> {
-        Err(unimpl())
-    }
 }

src/librustrt/rtio.rs

@@ -233,8 +233,6 @@ fn spawn(&mut self, cfg: ProcessConfig)
     fn pipe_open(&mut self, fd: c_int) -> IoResult<Box<RtioPipe + Send>>;
     fn tty_open(&mut self, fd: c_int, readable: bool)
             -> IoResult<Box<RtioTTY + Send>>;
-    fn signal(&mut self, signal: int, cb: Box<Callback + Send>)
-        -> IoResult<Box<RtioSignal + Send>>;
 }
 
 pub trait RtioTcpListener : RtioSocket {