Remove dead C code

#include <openssl/dh.h>

#include <openssl/bn.h>

#if defined(__APPLE__) || defined(__linux)

#include <pthread.h>

#include <openssl/crypto.h>

unsigned long thread_id()

{

    return (unsigned long) pthread_self();

}

void rust_openssl_set_id_callback() {

  CRYPTO_set_id_callback(thread_id);

}

#else

// Openssl already handles Windows directly, so we don't

// need to explicitly set it

void rust_openssl_set_id_callback() {

  // We don't know how to set the callback for arbitrary OSes

  // Let openssl use its defaults and hope they work.

}

#endif

#if OPENSSL_VERSION_NUMBER < 0x10000000L

// Copied from openssl crypto/hmac/hmac.c

int HMAC_CTX_copy(HMAC_CTX *dctx, HMAC_CTX *sctx)