tests: fix implied_bounds_in_impls clippy warn

```
error: this bound is already specified as the supertrait of `DerefMut`
   --> rustls/tests/./common/mod.rs:596:35
    |
596 |     client: &mut (impl DerefMut + Deref<Target = ConnectionCommon<impl SideData>>),
    |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#implied_bounds_in_impls
help: try removing this bound
    |
596 -     client: &mut (impl DerefMut + Deref<Target = ConnectionCommon<impl SideData>>),
596 +     client: &mut (impl DerefMut<Target = ConnectionCommon<impl SideData>>),
```

rustls/tests/common/mod.rs

@@ -2,7 +2,7 @@
 #![cfg(any(feature = "ring", feature = "aws_lc_rs"))]
 
 use std::io;
-use std::ops::{Deref, DerefMut};
+use std::ops::DerefMut;
 use std::sync::Arc;
 
 use pki_types::{
@@ -153,8 +153,8 @@ embed_files! {
 }
 
 pub fn transfer(
-    left: &mut (impl DerefMut + Deref<Target = ConnectionCommon<impl SideData>>),
-    right: &mut (impl DerefMut + Deref<Target = ConnectionCommon<impl SideData>>),
+    left: &mut impl DerefMut<Target = ConnectionCommon<impl SideData>>,
+    right: &mut impl DerefMut<Target = ConnectionCommon<impl SideData>>,
 ) -> usize {
     let mut buf = [0u8; 262144];
     let mut total = 0;
@@ -182,7 +182,7 @@ pub fn transfer(
     total
 }
 
-pub fn transfer_eof(conn: &mut (impl DerefMut + Deref<Target = ConnectionCommon<impl SideData>>)) {
+pub fn transfer_eof(conn: &mut impl DerefMut<Target = ConnectionCommon<impl SideData>>) {
     let empty_buf = [0u8; 0];
     let empty_cursor: &mut dyn io::Read = &mut &empty_buf[..];
     let sz = conn.read_tls(empty_cursor).unwrap();
@@ -621,8 +621,8 @@ pub fn make_pair_for_arc_configs(
 }
 
 pub fn do_handshake(
-    client: &mut (impl DerefMut + Deref<Target = ConnectionCommon<impl SideData>>),
-    server: &mut (impl DerefMut + Deref<Target = ConnectionCommon<impl SideData>>),
+    client: &mut impl DerefMut<Target = ConnectionCommon<impl SideData>>,
+    server: &mut impl DerefMut<Target = ConnectionCommon<impl SideData>>,
 ) -> (usize, usize) {
     let (mut to_client, mut to_server) = (0, 0);
     while server.is_handshaking() || client.is_handshaking() {