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.

Author: thaJeztah

Diff for: 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
 

Diff for: 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"
 

Diff for: blake2b/blake2b_amd64.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 (