If you used "dnf system-upgrade", then F23 should be pretty much gone. You may have some leftover fc23 kernels in /boot, and you can either manually uninstall them or just wait for F25 kernel upgrades to boot them out automatically.
rpm -qa | grep fc23
may show a few rpm packages, but be careful here, as some may still be relevant. Some may be just orphaned. You can check /var/ for any leftover F23 yum/dnf directories, and remove them if you find them still there.