You can't delete it per-se, but if you export to any image file it will get rid of it. Exporting to PDF, and then even opening that pdf in photoshop to get rid of the watermark is another method, and...
When you delete something you will get rid of it and all of the stuff inside. And anyway why would you delete something when you could sell it and get money out of it?
Go to the YouTube video I am linking to below. Or search for "file won't delete" in the search there, more than one way to do it. First of all, I've tried deleting it by dragging it to the recycling...