add prune and remove unused packages

vendor/golang.org/x/sys/plan9/str.go

@@ -1,22 +0,0 @@
-// Copyright 2009 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.
-
-// +build plan9
-
-package plan9
-
-func itoa(val int) string { // do it here rather than with fmt to avoid dependency
-	if val < 0 {
-		return "-" + itoa(-val)
-	}
-	var buf [32]byte // big enough for int64
-	i := len(buf) - 1
-	for val >= 10 {
-		buf[i] = byte(val%10 + '0')
-		i--
-		val /= 10
-	}
-	buf[i] = byte(val + '0')
-	return string(buf[i:])
-}