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/miner.cpp

@@ -10,8 +10,6 @@
 #include "net.h"
 #include "wallet.h"
 
-#include <stdint.h>
-
 double dHashesPerSec = 0.0;
 int64_t nHPSTimerStart = 0;