Image: don't re-queue thumbnails when deleting them

models/Image.php

@@ -160,8 +160,7 @@ class Image extends Asset
 		}
 
 		return Registry::get('db')->query('
-			UPDATE assets_thumbs
-			SET filename = NULL
+			DELETE FROM assets_thumbs
 			WHERE id_asset = {int:id_asset}',
 			['id_asset' => $this->id_asset]);
 	}