net: remove PollEvented noise from Debug for sockets

[seanmonstar]

This cleans up the debug output of println!("{:?}", tcp) from this:

// before
PollEvented {
    io: Some(
        TcpStream {
            addr: 1.2.3.4:36304,
            peer: 4.3.2.1:80,
            fd: 6,
        },
    ),
}

To this:

// after
TcpStream {
    addr: 1.2.3.4:36304,
    peer: 4.3.2.1:80,
    fd: 6,
}

[Darksonn]

Nice! Any chance you could do the same to the other sockets?

[seanmonstar]

Updated for tcp, unix, and windows types that had a similar Debug impl.

[Darksonn]

Thanks.