cmd/initContainer: Do not try to remount /etc/machine-id as ro

Followup to 1cc9e07

Sometimes the parent location might be mounted with
nosuid,nodev,noexec and trying to remount it as ro would remove
those and thus fails.

See commit mentioned above for more details.

containers#911

src/cmd/initContainer.go

@@ -52,7 +52,7 @@ var (
 		source        string
 		flags         string
 	}{
-		{"/etc/machine-id", "/run/host/etc/machine-id", "ro"},
+		{"/etc/machine-id", "/run/host/etc/machine-id", ""},
 		{"/run/libvirt", "/run/host/run/libvirt", ""},
 		{"/run/systemd/journal", "/run/host/run/systemd/journal", ""},
 		{"/run/systemd/resolve", "/run/host/run/systemd/resolve", ""},