Remove code intended for Go 1.4 and older.

This change only removes optimizations for older Go releases that we
no longer support in the internal/zero package.  This package should
still continue to build on older releases.

internal/zero/slice.go

@@ -3,10 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // This file implements range-based zeroing, which as of Go 1.5 is
-// optimized using a Duff's device.  See slice14.go for an implementation
+// optimized using a Duff's device.
-// optimized for previous Go versions.
-//
-// +build !go1.3,!go1.4
 
 package zero
 

internal/zero/slice14.go

@@ -1,50 +0,0 @@
-// Copyright (c) 2015 The btcsuite developers
-// Use of this source code is governed by an ISC
-// license that can be found in the LICENSE file.
-
-// Go >= 1.5 optimizes range-based zeroing of the form:
-//
-//   for i := range slice {
-//           slice[i] = 0
-//   }
-//
-// to an optimized implementation using a Duff's device, but older versions
-// do not and benefit from this custom implementation.
-//
-// +build go1.3 go1.4
-
-package zero
-
-import (
-	"math/big"
-)
-
-// Bytes sets all bytes in the passed slice to zero.  This is used to
-// explicitly clear private key material from memory.
-//
-// In general, prefer to use the fixed-sized zeroing functions (Bytea*)
-// when zeroing bytes as they are much more efficient than the variable
-// sized zeroing func Bytes.
-func Bytes(b []byte) {
-	z := [32]byte{}
-	n := uint(copy(b, z[:]))
-	for n < uint(len(b)) {
-		copy(b[n:], b[:n])
-		n <<= 1
-	}
-}
-
-// BigInt sets all bytes in the passed big int to zero and then sets the
-// value to 0.  This differs from simply setting the value in that it
-// specifically clears the underlying bytes whereas simply setting the value
-// does not.  This is mostly useful to forcefully clear private keys.
-func BigInt(x *big.Int) {
-	b := x.Bits()
-	z := [16]big.Word{}
-	n := uint(copy(b, z[:]))
-	for n < uint(len(b)) {
-		copy(b[n:], b[:n])
-		n <<= 1
-	}
-	x.SetInt64(0)
-}