core: remove includes in .cpp, if header is already in .h

- example: if util.h includes stdint.h, remove it from util.cpp, as
  util.h is the first header included in util.cpp

src/bitcoinrpc.cpp

@@ -11,8 +11,6 @@
 #include "util.h"
 #include "wallet.h"
 
-#include <stdint.h>
-
 #include <boost/algorithm/string.hpp>
 #include <boost/asio.hpp>
 #include <boost/asio/ssl.hpp>