Currently, we store the network_transaction_id for the setup_mandate/off_session payments only if the is_connector_agnostic_mit_enabled config is enabled. During the MITs, we refer to this flag to decide whether to use the connector_mandate_id or the network_transaction_id for the MIT.

Instead of using the flag for multiple purposes, it should be used solely to determine whether to use the connector_mandate_id or the network_transaction_id for the MIT. Therefore, this change will ensure that the network_transaction_id is always stored for off-session payments if it is present in the connector response.