KVM: s390: Removing untriggerable BUG_ONs

The BUG_ON in kvm-s390.c is unreachable, as we get the vcpu per common code,
which itself does this from the private_data field of the file descriptor,
and there is no KVM_UNCREATE_VCPU.

The __{set,unset}_cpu_idle BUG_ONs are not triggerable because the vcpu
creation code already checks against KVM_MAX_VCPUS.

Signed-off-by: Dominik Dingel <dingel@linux.vnet.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>