You can't. The Photos Library file contains all your photos. If you delete it, it will take all the photos inside it with it.
If you moved the photos library to the desktop and the Photos app iOS now loading it from there, then deleting it would be a bad choice. You would want to return it to its original location if that's the case.
If you made a copy of the photos library on the desktop, then deleting it won't affect the photos in the other library that is actively being used by the Photos App.
To verify what Photos Library the Photos App is actively using, open it, and the go to the Photos Menu ➜ Settings ➜ General ➜ Library Location, to see which library file its loading. If it's the one on the desktop, then you don't want to delete it. If it's using some other library file there and you are confident all your photos are there in the Photos app, then deleting the library from the desktop won't affect them in any way.
In other words, unless the Photos app is actively using that library, deleting it won't affect your photos. It will affect the photos in that specific library.