Removes unused containers included in headers.

This was SVN commit r25004.

source/simulation2/system/ParamNode.h

@@ -1,4 +1,4 @@
-/* Copyright (C) 2017 Wildfire Games.
+/* Copyright (C) 2021 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -23,8 +23,9 @@
 #include "ps/CStrIntern.h"
 #include "ps/Errors.h"
 #include "scriptinterface/ScriptTypes.h"
+
 #include <map>
-#include <set>
+#include <string>
 
 class XMBFile;
 class XMBElement;