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/core.cpp

@@ -7,8 +7,6 @@
 
 #include "util.h"
 
-#include <stdint.h>
-
 std::string COutPoint::ToString() const
 {
     return strprintf("COutPoint(%s, %u)", hash.ToString().substr(0,10).c_str(), n);