What does "Deleting unrecoverable checkpoint table row" mean?
This is my assumption based on the log entry - 

If you are using Memory optimized tables, there are Checkpoint File Pair (CFPs).


  CPFs transition through several states before they can be deallocated. Database checkpoints and log backups need to happen to transition the files through the phases, and ultimately clean up files that are no longer needed. For a description of these phases, see sys.dm_db_xtp_checkpoint_files 


The log entry refers to deallocation by garbage collection process.