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/walletdb.cpp

@@ -12,7 +12,6 @@
 #include "wallet.h"
 
 #include <inttypes.h>
-#include <stdint.h>
 
 #include <boost/filesystem.hpp>
 #include <boost/foreach.hpp>