Remove an unused field in ossl_shim

  // packeted_bio is the packeted BIO which simulates read timeouts.

  BIO *packeted_bio = nullptr;

  bool cert_ready = false;

  bssl::UniquePtr<SSL_SESSION> session;

  bool handshake_done = false;

  // private_key is the underlying private key used when testing custom keys.

  bssl::UniquePtr<EVP_PKEY> private_key;