Unable to truncate transaction log, log_reuse_wait_desc - AVAILABILITY_REPLICA

If you do this:

SELECT * FROM sys.databases

And the log_reuse_wait_desc shows AVAILABILITY_REPLICA, that means SQL Server is waiting to send log data to one of your Always On Availability Group replicas. One of the replicas may be lagging behind due to a slow network, or it may be down altogether.

If you check the AG dashboard and it shows no queues, you may have been a victim of thread exhaustion. It's a known issue that the AG dashboard stops updating after worker thread exhaustion. You'll need to check the status on each replica directly rather than relying on the primary. Nick's note in that Connect item says that you can just alter a replica's properties to restart replication, but that doesn't always work (especially if you have hundreds of databases on a replica with a large amount of data that needs to be sent, and restarting replication can just cause the worker thread exhaustion again.)

If the last guy set up an AG replica and it's not supposed to exist anymore, then it's time to remove that AG and/or replica. Just be careful that apps aren't pointing to the listener name in order to connect to your SQL Server.