vmstate: remove unused declaration

Since 38e0735e, register_device_unmigratable() has been removed
Signed-off-by: NMarc-André Lureau <marcandre.lureau@gmail.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

include/migration/vmstate.h

@@ -75,9 +75,6 @@ int register_savevm_live(DeviceState *dev,
                          void *opaque);
 
 void unregister_savevm(DeviceState *dev, const char *idstr, void *opaque);
-void register_device_unmigratable(DeviceState *dev, const char *idstr,
-                                                                void *opaque);
-
 
 typedef struct VMStateInfo VMStateInfo;
 typedef struct VMStateDescription VMStateDescription;