In your scenario you have a 2 node AlwaysOn AG. You first need to remove the Availability Group.
DROP AVAILABILITY GROUP MyAGNameHere
This will deconstruct the AG relationship between the nodes and allow you to recover the databases on the secondary. If you remove a node from the cluster without doing this, you will just start getting errors about the AG. Once the AG has been removed, recover the databases on the secondary. Now you will have two standalone SQL Servers you can use. They will still be in a Windows Cluster. If you want to get rid of the cluster itself, you can do that next. You an use the Remove-Clusternode as previously suggested, but that doesn't remove the Windows cluster itself, it just removes a node from the existing cluster.
You can then remove the cluster with this example:
Get-Cluster -Name Cluster1 | Remove-Cluster -Force -CleanupAD .
If these machines were only for this one application, then you could remove the nodes and destroy the cluster, leaving yourself with two stand alone nodes, then set up Log Shipping for some sort of recoverability/manual failover if you need or want that.
The other links in the previous answer are referring to a SQL Server Failover Cluster Instance, which is NOT what you have. I see a lot of confusion about that. A SQL Server Failover Cluster is different from an AlwaysOn Availability Group.