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

sandbox/mac/xpc_private_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 private XPC functions. This file is
-// used for both forward declarations of private symbols and to use with
-// tools/generate_stubs for creating a dynamic library loader.
-
-// Dictionary manipulation.
-void xpc_dictionary_set_mach_send(xpc_object_t dictionary, const char* name, mach_port_t port);
-void xpc_dictionary_get_audit_token(xpc_object_t dictionary, audit_token_t* token);
-
-// Raw object getters.
-mach_port_t xpc_mach_send_get_right(xpc_object_t value);
-
-// Pipe methods.
-xpc_pipe_t xpc_pipe_create_from_port(mach_port_t port, int flags);
-int xpc_pipe_receive(mach_port_t port, xpc_object_t* message);
-int xpc_pipe_routine(xpc_pipe_t pipe, xpc_object_t request, xpc_object_t* reply);
-int xpc_pipe_routine_reply(xpc_object_t reply);
-int xpc_pipe_simpleroutine(xpc_pipe_t pipe, xpc_object_t message);
-int xpc_pipe_routine_forward(xpc_pipe_t forward_to, xpc_object_t request);