all: replace io/ioutil with io and os package

For golang/go#45557

Change-Id: I447530cc66896aef7a8d528ccb8d095b80e3cf47
GitHub-Last-Rev: 5f385ff46487ac318bd1147cdbbd26bb0ffd0426
GitHub-Pull-Request: golang/crypto#230
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/430797
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Meng Zhuo <mzh@golangcn.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

ssh/cipher.go

@@ -15,7 +15,6 @@ import (
 	"fmt"
 	"hash"
 	"io"
-	"io/ioutil"
 
 	"golang.org/x/crypto/chacha20"
 	"golang.org/x/crypto/internal/poly1305"
@@ -497,7 +496,7 @@ func (c *cbcCipher) readCipherPacket(seqNum uint32, r io.Reader) ([]byte, error)
 			// data, to make distinguishing between
 			// failing MAC and failing length check more
 			// difficult.
-			io.CopyN(ioutil.Discard, r, int64(c.oracleCamouflage))
+			io.CopyN(io.Discard, r, int64(c.oracleCamouflage))
 		}
 	}
 	return p, err