I recently had to remove an AlwaysOn Availability Group configuration because the application did not support any solution that depends on Windows Failover Clustering. Here are the steps I took:
1. Deleted the availability group from SQL Server Management Studio’s Object Explorer
2. Disabled AlwaysOn Availability Groups from SQL Server Configuration manager.
NB: If you do step two before step one, you will not be able to open any of the replicas. You will need to re-enable AlwaysOn AG Groups from SSCM and then start again from step one.
3. Destroyed the cluster from Failover Cluster Manager.
4. Cleaned up the cluster nodes using the Clear-ClusterNode command on PowerShell. It is important to run PowerShell as an administrator when doing this.