mac: Remove some 10.6-SDK-only sandbox code.

sandbox/mac/xpc_stubs.sig

-// Copyright 2014 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.
-
-// This file contains declarations of public XPC functions used in the sandbox.
-// This file is used with tools/generate_stubs for creating a dynamic library
-// loader.
-
-// XPC object management.
-void xpc_release(xpc_object_t object);
-
-// Dictionary manipulation.
-xpc_object_t xpc_dictionary_create(const char* const *keys, const xpc_object_t* values, size_t count);
-const char* xpc_dictionary_get_string(xpc_object_t dictionary, const char* key);
-uint64_t xpc_dictionary_get_uint64(xpc_object_t dictionary, const char* key);
-void xpc_dictionary_set_uint64(xpc_object_t dictionary, const char* key, uint64_t value);
-int64_t xpc_dictionary_get_int64(xpc_object_t dictionary, const char* key);
-void xpc_dictionary_set_int64(xpc_object_t dictionary, const char* key, int64_t value);
-bool xpc_dictionary_get_bool(xpc_object_t dictionary, const char* key);
-xpc_object_t xpc_dictionary_create_reply(xpc_object_t request);