Remove usage of USE_SYSTEM_PROTOBUF

It's no longer possible to build using the system libprotobuf, so the
macro USE_SYSTEM_PROTOBUF is not set anywhere.  This CL removes all
usages of it.

R=jam@chromium.org

Review-Url: https://codereview.chromium.org/2748203002
Cr-Commit-Position: refs/heads/master@{#462363}


CrOS-Libchrome-Original-Commit: 4b7c48089ce5ee9d1a2e4530a82f6bd31e398bbd
diff --git a/dbus/message.cc b/dbus/message.cc
index c8663f7..b33345e 100644
--- a/dbus/message.cc
+++ b/dbus/message.cc
@@ -13,12 +13,7 @@
 #include "base/strings/string_util.h"
 #include "base/strings/stringprintf.h"
 #include "dbus/object_path.h"
-
-#if defined(USE_SYSTEM_PROTOBUF)
-#include <google/protobuf/message_lite.h>
-#else
 #include "third_party/protobuf/src/google/protobuf/message_lite.h"
-#endif
 
 namespace {