Remove Documentation/virtual/virtio-spec.txt

We haven't been keeping it in sync, so just remove it.
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

Documentation/virtual/00-INDEX

@@ -6,6 +6,3 @@ kvm/
 	- Kernel Virtual Machine.  See also http://linux-kvm.org
 uml/
 	- User Mode Linux, builds/runs Linux kernel as a userspace program.
-virtio.txt
-	- Text version of draft virtio spec.
-          See http://ozlabs.org/~rusty/virtio-spec