message: Remove mention of ec_privkey_export because it doesn't exist
  Summary: This is a backport of libsecp256k1 PR664
  Test Plan:
  ninja check-secp256k1
  Reviewers: #bitcoin_abc, Fabien
diff: @@ -170,7 +170,7 @@ typedef int (*secp256k1_nonce_function)(
  #define SECP256K1_CONTEXT_SIGN (SECP256K1_FLAGS_TYPE_CONTEXT | SECP256K1_FLAGS_BIT_CONTEXT_SIGN)
  #define SECP256K1_CONTEXT_NONE (SECP256K1_FLAGS_TYPE_CONTEXT)
  -/** Flag to pass to secp256k1_ec_pubkey_serialize and secp256k1_ec_privkey_export. */
  +/** Flag to pass to secp256k1_ec_pubkey_serialize. */
  #define SECP256K1_EC_COMPRESSED (SECP256K1_FLAGS_TYPE_COMPRESSION | SECP256K1_FLAGS_BIT_COMPRESSION)
  #define SECP256K1_EC_UNCOMPRESSED (SECP256K1_FLAGS_TYPE_COMPRESSION)
  