kvm/s390: drop unpaired smp_mb

smp_mb on vcpu destroy isn't paired with anything, violating pairing
rules, and seems to be useless.

Drop it.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <1452010811-25486-1-git-send-email-mst@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>