Remove usage of USE_SYSTEM_PROTOBUF

remoting/protocol/message_serialization.h

Index: remoting/protocol/message_serialization.h
diff --git a/remoting/protocol/message_serialization.h b/remoting/protocol/message_serialization.h
index cebc5aafb8fae33471b013700726013f84e59fb7..f05cae7bd6633b0becfe9c6509859582ccb1fafe 100644
--- a/remoting/protocol/message_serialization.h
+++ b/remoting/protocol/message_serialization.h
@@ -10,12 +10,7 @@
 
 #include "net/base/io_buffer.h"
 #include "remoting/base/compound_buffer.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 remoting {
 namespace protocol {