cmd/internal/obj: remove unused ar.go

Change-Id: I3eab54cd27c43bab38641fbead4adb6442481356
Reviewed-on: https://go-review.googlesource.com/20248
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/internal/obj/ar.go

-// 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.
-
-package obj
-
-type ar_hdr struct {
-	name string
-	date string
-	uid  string
-	gid  string
-	mode string
-	size string
-	fmag string
-}