blake2b: drop Go 1.6, Go 1.8 compatibility

Other packages already dropped compatibility with go < 1.12, so it
should be safe to remove it for this package as well.

Change-Id: I7e894fd11d2e7d1fe28c647bd921399a9a6e30d0
GitHub-Last-Rev: 2b4f576
GitHub-Pull-Request: #240
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/448240
Run-TryBot: Nicola Murino <nicola.murino@gmail.com>
Reviewed-by: Joedian Reid <joedian@golang.org>
Reviewed-by: Nicola Murino <nicola.murino@gmail.com>
Commit-Queue: Nicola Murino <nicola.murino@gmail.com>
Auto-Submit: Nicola Murino <nicola.murino@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

blake2b/blake2bAVX2_amd64.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.7 && amd64 && gc && !purego
+//go:build amd64 && gc && !purego
 
 package blake2b
 

blake2b/blake2bAVX2_amd64.s

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.7 && amd64 && gc && !purego
+//go:build amd64 && gc && !purego
 
 #include "textflag.h"
 

blake2b/register.go

@@ -2,8 +2,6 @@
 // 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 blake2b
 
 import (