Fix grave kio bug with FTP: if you delete a symlink to a folder, it would recursively delete the folder (!)

Sounds stupid, but it's FTP which is stupid; the only way to know if something is a link is by listing its parent dir, but this isn't always possible (if no permission to list the parent dir)... -> the URL isn't enough info, we need the KFileItem from the listing. Rather than adding KIO::del(KFileItemList) (and porting a lot of code), I simply grab the KFileItem from KDirListerCache, as discussed with Michael Brade in 2006 for other reasons.

As a side effect, this speeds up deletion (again!) in the case of remote protocols (saves a KIO::stat per toplevel item to delete).