That's normal behavior. They are never removed and there are no Periodic script or cron job that cleans them up. They are preserved in case you need to do a system rollback.for some reason after upgrading to 14.4 the files weren't deleted automagically.
Once you've tested that everything is running smoothly, you can remove everything in there:Can I safely delete the files in the bigger dirs?
# rm -r /var/db/freebsd-update/*