Remove legacy install-arm-sysroot.py script

This was merged with install-debian.wheezy.sysroot.py a
long time ago.

Review URL: https://codereview.chromium.org/1050983003

Cr-Commit-Position: refs/heads/master@{#323944}

build/linux/install-arm-sysroot.py

-#!/usr/bin/env python
-# Copyright (c) 2012 The Chromium Authors. All rights reserved.
-# Use of this source code is governed by a BSD-style license that can be
-# found in the LICENSE file.
-
-import sys
-
-msg = '''\
-ERROR: This script has merged with install-debian.wheezy.sysroot.py.
-Please use that instead (with --arch=arm).
-'''
-sys.stderr.write(msg)