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/netbase.cpp

@@ -10,8 +10,6 @@
 #include "uint256.h"
 #include "util.h"
 
-#include <stdint.h>
-
 #ifndef WIN32
 #include <fcntl.h>
 #endif