What should I do with an old, unused septic tank in my yard?
You'll have to check codes, but I believe the usual procedure is to fill them with something like sand - since it's a septic tank, and since (I presume) you had it emptied before leaving it to rot, you probably don't need to clean it, just get it filled.

You can NOT just leave it there - eventually it will rot away and collapse, presenting a hazard at some unknown future time.

You might call around to the local septic companies and ask them what the usual procedure is in your area, they should know who can handle the fill-in of the tank properly.