remove old TODO

zfs_backup.py

@@ -680,7 +680,6 @@ class ExternalPool(Pool):
             exit(EXIT_ERROR)
 
     def clean_old_snapshots(self) -> None:
-        # this  seems to be broken  TODO
         print("Cleaning old snapshots")
         for dataset in self.datasets:
             dataset.clean_old_snapshots()