Revert "add yield to loop for udp relay down stream speed low"

This reverts commit fd4f1f1.

crates/shadowsocks-service/src/local/socks/server/socks5/udprelay.rs

@@ -173,7 +173,6 @@ impl Socks5UdpServer {
                             err
                         );
                     }
-                    tokio::task::yield_now().await
                 }
             }
         }

crates/shadowsocks-service/src/server/udprelay.rs

@@ -185,7 +185,6 @@ impl UdpServer {
                             // If Result is error, the channel receiver is closed. We should exit the task.
                             break;
                         }
-                        tokio::task::yield_now().await
                     }
                 }));
             }