KVM: s390: remove outdated documentation

The old Documentation/s390/kvm.txt file is either
outdated or described in Documentation/virtual/kvm/api.txt.
Let's get rid of it.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>