[clean up] Add presence validation for object in SnapshotItem model

lib/active_snapshot/models/snapshot_item.rb

@@ -14,8 +14,7 @@ class SnapshotItem < ActiveRecord::Base
     validates :item_type, presence: true
     validates :object, presence: true
 
-    def object(raw: false)
-      return super() if raw
+    def object
       return @object if @object
 
       if ActiveSnapshot.config.storage_method_json?