Remove unused includes of boost lexical_cast

We don't use lexical_cast anywhere, no need to include it.

src/rpcdump.cpp

@@ -14,7 +14,6 @@
 
 #include <boost/algorithm/string.hpp>
 #include <boost/date_time/posix_time/posix_time.hpp>
-#include <boost/lexical_cast.hpp>
 #include "json/json_spirit_value.h"
 
 using namespace json_spirit;