qemu: Remove now obsolete assignment of default network card model for s390 hosts

This effectively reverts commit 539d73db as the
changes aren't needed after introduction of the XML post parse callbacks.

src/qemu/qemu_command.c

@@ -919,11 +919,6 @@ qemuDomainPrimeS390VirtioDevices(virDomainDefPtr def,
     }
 
     for (i = 0; i < def->nnets ; i++) {
-        if ((def->os.arch == VIR_ARCH_S390 ||
-             def->os.arch == VIR_ARCH_S390X) &&
-            def->nets[i]->model == NULL) {
-            def->nets[i]->model = strdup("virtio");
-        }
         if (STREQ(def->nets[i]->model,"virtio") &&
             def->nets[i]->info.type == VIR_DOMAIN_DEVICE_ADDRESS_TYPE_NONE) {
             def->nets[i]->info.type = type;