Revert "util: upgrade tokio-util to bytes 0.6 (#3052)" (#3060)

This reverts commit fe2b997.

We are avoiding adding poll_read_buf to tokio itself for now. The patch is
reverted now in order to not block the v0.3.2 release (#3059).

tokio/tests/io_read_buf.rs

@@ -4,7 +4,6 @@
 use tokio::io::{AsyncRead, AsyncReadExt, ReadBuf};
 use tokio_test::assert_ok;
 
-use futures::future::poll_fn;
 use std::io;
 use std::pin::Pin;
 use std::task::{Context, Poll};
@@ -35,38 +34,3 @@ async fn read_buf() {
     assert_eq!(n, 11);
     assert_eq!(buf[..], b"hello world"[..]);
 }
-
-#[tokio::test]
-async fn poll_read_buf() {
-    struct Rd {
-        cnt: usize,
-    }
-
-    impl AsyncRead for Rd {
-        fn poll_read(
-            mut self: Pin<&mut Self>,
-            _cx: &mut Context<'_>,
-            buf: &mut ReadBuf<'_>,
-        ) -> Poll<io::Result<()>> {
-            self.cnt += 1;
-            buf.put_slice(b"hello world");
-            Poll::Ready(Ok(()))
-        }
-    }
-
-    let mut buf = vec![];
-    let mut rd = Rd { cnt: 0 };
-
-    let res = tokio::spawn(async move {
-        poll_fn(|cx| {
-            let res = rd.poll_read_buf(&mut buf, cx);
-            assert_eq!(1, rd.cnt);
-            assert_eq!(buf[..], b"hello world"[..]);
-            res
-        })
-        .await
-    })
-    .await;
-
-    assert!(matches!(res, Ok(Ok(11usize))));
-}