Remove the deprecation warning for get_session_data_subset
It seems that get_session_data_subset was marked deprecated in favor of dump_from_session_manager, but dump_from_session_manager actually calls get_session_data_subset and then just dumps it to a stream. Since I'm guessing there could be some cases where we might want this as a return value rather than a stream, I'm proposing that we remove the deprecation warning.