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-util/src/io/stream_reader.rs

@@ -1,4 +1,4 @@
-use bytes::{Buf, BufMut};
+use bytes::Buf;
 use futures_core::stream::Stream;
 use pin_project_lite::pin_project;
 use std::io;
@@ -119,29 +119,6 @@ where
         self.consume(len);
         Poll::Ready(Ok(()))
     }
-    fn poll_read_buf<BM: BufMut>(
-        mut self: Pin<&mut Self>,
-        cx: &mut Context<'_>,
-        buf: &mut BM,
-    ) -> Poll<io::Result<usize>>
-    where
-        Self: Sized,
-    {
-        if !buf.has_remaining_mut() {
-            return Poll::Ready(Ok(0));
-        }
-
-        let inner_buf = match self.as_mut().poll_fill_buf(cx) {
-            Poll::Ready(Ok(buf)) => buf,
-            Poll::Ready(Err(err)) => return Poll::Ready(Err(err)),
-            Poll::Pending => return Poll::Pending,
-        };
-        let len = std::cmp::min(inner_buf.len(), buf.remaining_mut());
-        buf.put_slice(&inner_buf[..len]);
-
-        self.consume(len);
-        Poll::Ready(Ok(len))
-    }
 }
 
 impl<S, B, E> AsyncBufRead for StreamReader<S, B>