You would remove the AutoKey IKE object first, I suspect this is still there.
Then remove the tunnel binding from the Autokey Advanced > Gateway
Then remove the Autokey Advanced > gateway object itself.
You may also have routes pointing towards the tunnel interface
Nework > Routing > Destination
Then when these are gone you should be able to remove the no longer used tunnel interface itself
Network > Interfaces > List
You will probably also have security rules in Policy > Policies that will then no longer be needed that allowed the traffic