You can't get rid of timestamp because folder names MUST be unique, so all backtest can be stored and retrieved later and also because AmiBroker is MULTITHREADED and many threads may write at the same time.
If folder was shared, there would be conflicts accessing same file leading to slowdowns/lockups/failures.
If you don't like that, you can always write a simple batch that would delete and/or rename folders that you don't need.