Solaris 11.3 still doesn't support dumpadm -d none. This is useful in some scenarios, for example when troubleshooting failed AI installations when you try to restart it withouth rebooting. This will generally fail as it won't be able to destroy rpool as there is a dump device already configured there.
There is a workaround though.
Edit /etc/dumpadm.conf file and comment out line containing DUMPADM_DEVICE, then run dumpadm -u. This will unconfigure dump device entirely. Then just run zpool destroy rpool and now you can svcadm clear auto-installer.