Based on your description, Lightroom isn't using the NEF files, it is only using the DNG files.
The NEF files contain information from the camera that is not transferred to the DNG files. This information could be useful if someday you are going to use some other software than Lightroom to process these files. So throwing them away will not harm Lightroom, but it will cause a loss of information. Only you can decide if this loss of information is acceptable ... however, I do point out that obtaining an additional hard drive and not throwing anything away seems a preferable solution to me.
however, I do point out that obtaining an additional hard drive and not throwing anything away seems a preferable solution to me.
heidie22, dj_paige has a very good point there. I would not through away the NEF files; I would archive them to a external drive.
By the way, I am assuming that converting RAW files to DNG is a decision you have made and not an accident.