io: remove poll_{read,write}_buf from traits (#2882)

These functions have object safety issues. It also has been decided to
avoid vectored operations on the I/O traits. A later PR will bring back
vectored operations on specific types that support them.

Refs: #2879, #2716

tokio-test/src/io.rs

@@ -22,7 +22,6 @@ use tokio::io::{AsyncRead, AsyncWrite, ReadBuf};
 use tokio::sync::mpsc;
 use tokio::time::{self, Delay, Duration, Instant};
 
-use bytes::Buf;
 use futures_core::ready;
 use std::collections::VecDeque;
 use std::future::Future;
@@ -439,16 +438,6 @@ impl AsyncWrite for Mock {
         }
     }
 
-    fn poll_write_buf<B: Buf>(
-        self: Pin<&mut Self>,
-        cx: &mut task::Context<'_>,
-        buf: &mut B,
-    ) -> Poll<io::Result<usize>> {
-        let n = ready!(self.poll_write(cx, buf.bytes()))?;
-        buf.advance(n);
-        Poll::Ready(Ok(n))
-    }
-
     fn poll_flush(self: Pin<&mut Self>, _cx: &mut task::Context<'_>) -> Poll<io::Result<()>> {
         Poll::Ready(Ok(()))
     }