I don't know where this topic fits exactly so I will take a guess here. I ran a registry scan after I removed windows defender from my computer. I got two things that came up that were defender dll files. a screenshot is attached below as to what those two file paths are.. the only one that I am wondering about is the second entry which is the @C:\ instead of the C:\Program Files location like the other one is.
Since Windows Defender is no longer installed I'd say it's safe to delete. If in doubt make a system restore point, and then when CCleaner offers to make a backup before deleting the obsolete registry data allow it to do so. You may also have other Windows Defender stuff lingering on your computer too, as in data files.