Remove serialisation methods of DeviceGrant class

It seems useless to think of serialisation of DeviceGrant objects,
since they are by design ephemeral. The equivalent "grant" classes
for other grant_types, such as "class Grant(AbstractGrant)" does
not have these methods defined either. Therefore, best to remove
them.

Author: cristiprg

oauth2_provider/models.py

@@ -713,20 +713,10 @@ def is_expired(self):
         return False
 
 
-class DeviceManager(models.Manager):
-    def get_by_natural_key(self, client_id, device_code, user_code):
-        return self.get(client_id=client_id, device_code=device_code, user_code=user_code)
-
-
 class DeviceGrant(AbstractDeviceGrant):
-    objects = DeviceManager()
-
     class Meta(AbstractDeviceGrant.Meta):
         swappable = "OAUTH2_PROVIDER_DEVICE_GRANT_MODEL"
 
-    def natural_key(self):
-        return (self.client_id, self.device_code, self.user_code)
-
 
 @dataclass
 class DeviceRequest: