I have looked into this further, and actually if Delete Duplicates finds some duplicates and this leads to a folder that contained music files and jpegs (or other images) to only contain jpegs after the duplicates were deleted it will already delete the folder as you requested.
So I think in your case maybe you were left with a folder just containing artwork after running Fix Songs. So therefore Delete Duplicates didn’t find any files to delete in the folder and hence didnt delete it. Now as long as Fix Songs has Save:If song moved to another folder set to Everything then Fix Songs should move artwork with the songs and then delete the empty folder, but maybe there is some issue.
Going back to Delete Duplicates, although it will delete a folder it has made empty it does not delete the parent folder if that is now empty. I have now modified the code so it will delete the parents folder if now empty, and will also delete the grandparents folder if that is now empty, ready for next release - https://jthink.atlassian.net/browse/SONGKONG-2130