Can I delete files that have no downloads?

As you have already been told, there is no benefit. Besides, it is a lot of work.

Yes. You can delete old unsold assets, and it won’t affect your account. However, there’s really no benefit in doing so. I regularly earn first time sales from old files, so I’m not deleting anything. 

Yes, you can delete files. I don’t bother myself, as I still get an occasional sale out of the blue from assets I upload 4 years ago.