Remove num/gmp support

The whole "num" API and its libgmp-based implementation are now unused. Remove them.

src/bench_internal.c

@@ -10,7 +10,6 @@
 #include "assumptions.h"
 #include "util.h"
 #include "hash_impl.h"
-#include "num_impl.h"
 #include "field_impl.h"
 #include "group_impl.h"
 #include "scalar_impl.h"