spapr-cpu-core: Release ICPState object during CPU unrealization

Recent commits that re-organized ICPState object missed to destroy
the object when CPU is unrealized. Fix this so that CPU unplug
doesn't abort QEMU.

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>