ndctl/region.go: comment out call to block device nullify()

nullify() has been unreliable, so we implemented different
approach to clear device in Device Manager side.
We keep this call point for reference as commented out, just in case.

pkg/ndctl/region.go

@@ -281,9 +281,12 @@ func (r *Region) DestroyNamespace(ns *Namespace, force bool) error {
 		return nil
 	}
 
+	/* originally here we try to clear 4k at start of block device,
+	* but that seems not work reliably so we use different method via flushDevice
+	* This here remains commented out
 	if err := ns.nullify(); err != nil {
 		return fmt.Errorf("failed to nullify namespace: %s", err.Error())
-	}
+	}*/
 
 	C.ndctl_namespace_disable_invalidate(ndns)