Remove superfluous folder in archived submissions

Mahara 1.10+

When you archive submissions, the path to the actual file is like /var/lib/maharadata/110stable/submission/6/1414376651/ with "submission" being the folder for all submissions, "6" being the folder for the user and then the next subfolder being a timestamp for when the archive was created. Only then do you get to the actual file.

I think we can safely remove the timestamp folder as each submission has a serialized number at the end. Disadvantages of the timestamp folder:

1. It is not clear unless you are a dev that the number is a timestamp.
2. Even if you know it's a timestamp, you'd need to calculate first if it's the one file in that folder that you need or not.
3. It doesn't add value because the file names are unique.

Advantages of removing the folder:

1. You can see all archived files from one user in one folder in chronological order. That makes it easier to review the files quickly and find the ones you are looking for.
2. One level less of hierarchy.