bench_internal: Remove unnecessary includes

secp256k1.c is included anyway, so there's no need to include the
individual _impl.h files.

Author: real-or-random

src/bench_internal.c

@@ -10,13 +10,8 @@
 #include "../include/secp256k1.h"
 
 #include "assumptions.h"
-#include "util.h"
-#include "hash_impl.h"
-#include "field_impl.h"
-#include "group_impl.h"
-#include "scalar_impl.h"
-#include "ecmult_impl.h"
 #include "bench.h"
+#include "util.h"
 
 static void help(int default_iters) {
     printf("Benchmarks various internal routines.\n");