message: rpc: Simplify GetAllOutputTypes with the Join helper
  This commit does not change behavior
diff: @@ -503,12 +503,11 @@ static UniValue decoderawtransaction(const JSONRPCRequest& request)
  static std::string GetAllOutputTypes()
  {
  - std::string ret;
  + std::vector<std::string> ret;
  for (int i = TX_NONSTANDARD; i <= TX_WITNESS_UNKNOWN; ++i) {
  - if (i != TX_NONSTANDARD) ret += ", ";
  - ret += GetTxnOutputType(static_cast<txnouttype>(i));
  + ret.emplace_back(GetTxnOutputType(static_cast<txnouttype>(i)));
  }
  - return ret;
  + return Join(ret, ", ");
  }
  static UniValue decodescript(const JSONRPCRequest& request)
  