message: Remove unused field pskIdentity_ from ClientHandshake
  Summary:
  As per title.
  Pull Request resolved:
diff: @@ -197,8 +197,6 @@ class ClientHandshake : public Handshake {
  folly::exception_wrapper error_;
  - folly::Optional<std::string> pskIdentity_;
  -
  std::shared_ptr<CryptoFactory> cryptoFactory_;
  std::shared_ptr<ClientTransportParametersExtension> transportParams_;
  bool earlyDataAttempted_{false};
  