all: bump go.mod version and drop compatibility shims

Also, remove the legacy import annotations.

Fixes golang/go#68147

Change-Id: Ibfcc9322f27224c0ba92ea42cd56912a7d8783fd
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/594256
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>

blake2s/blake2s.go

@@ -16,9 +16,10 @@
 //
 // BLAKE2X is a construction to compute hash values larger than 32 bytes. It
 // can produce hash values between 0 and 65535 bytes.
-package blake2s // import "golang.org/x/crypto/blake2s"
+package blake2s
 
 import (
+	"crypto"
 	"encoding/binary"
 	"errors"
 	"hash"
@@ -55,6 +56,13 @@ func Sum256(data []byte) [Size]byte {
 // and BinaryUnmarshaler for state (de)serialization as documented by hash.Hash.
 func New256(key []byte) (hash.Hash, error) { return newDigest(Size, key) }
 
+func init() {
+	crypto.RegisterHash(crypto.BLAKE2s_256, func() hash.Hash {
+		h, _ := New256(nil)
+		return h
+	})
+}
+
 // New128 returns a new hash.Hash computing the BLAKE2s-128 checksum given a
 // non-empty key. Note that a 128-bit digest is too small to be secure as a
 // cryptographic hash and should only be used as a MAC, thus the key argument

blake2s/register.go

@@ -1,21 +0,0 @@
-// Copyright 2017 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-//go:build go1.9
-
-package blake2s
-
-import (
-	"crypto"
-	"hash"
-)
-
-func init() {
-	newHash256 := func() hash.Hash {
-		h, _ := New256(nil)
-		return h
-	}
-
-	crypto.RegisterHash(crypto.BLAKE2s_256, newHash256)
-}