Update comments referring to Chromium-side src/trusted/plugin code

SConstruct

 #! -*- python -*-
 # Copyright (c) 2012 The Native Client 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 atexit
 import json
 import os
 import platform
 import re
@@ skipping 2287 matching lines @@
 Common tasks:
 -------------
 
 * cleaning:           scons -c
 * building:           scons
 * build mandel:       scons --mode=nacl mandel.nexe
 * smoke test:         scons --mode=nacl,opt-linux -k pp=1 smoke_tests
 
 * sel_ldr:            scons --mode=opt-linux sel_ldr
 
-* build the plugin:         scons --mode=opt-linux ppNaClPlugin
-*      or:                  scons --mode=opt-linux src/trusted/plugin
-
 Targets to build trusted code destined for the SDK:
 * build trusted-code tools:     scons build_bin
 * install trusted-code tools:   scons install_bin bindir=...
 * These default to opt build, or add --mode=dbg-host for debug build.
 
 Targets to build untrusted code destined for the SDK:
 * build just libraries:         scons build_lib
 * install just headers:         scons install_headers includedir=...
 * install just libraries:       scons install_lib libdir=...
 * install headers and libraries:scons install includedir=... libdir=...
@@ skipping 1705 matching lines @@
   nacl_env.ValidateSdk()
 
 if BROKEN_TEST_COUNT > 0:
   msg = "There are %d broken tests." % BROKEN_TEST_COUNT
   if GetOption('brief_comstr'):
     msg += " Add --verbose to the command line for more information."
   print msg
 
 # separate warnings from actual build output
 Banner('B U I L D - O U T P U T:')