You didn’t find it because there is no autogen file. This is the name of the default retention policy. The retention policy is configured with an influxdb admin tool (you can do this e.g. with chronograf) or the influx cli. Not sure if there also is a config file to do this.
But more important: while you can have multiple retention policies for one database from influx side, the openhab binding supports just one retention policy. Means you can not choose to delete values earlier or later on a item level.