Removed taking care of bookshelf's changed model keys

no issue

- this is no longer needed for now
- it was anyway a little bit ugly to modify bookshelf's `changed` object
- if we want to change something about figuring out if a model has changed (including relations)
  -> we probably need to override bookshelf

core/server/models/base/index.js

@@ -180,18 +180,6 @@ ghostBookshelf.Model = ghostBookshelf.Model.extend({
         this.attributes = this.pick(this.permittedAttributes());
         // Store the previous attributes so we can tell what was updated later
         this._updatedAttributes = newObj.previousAttributes();
-
-        /**
-         * Bookshelf keeps none valid model attributes in `model.changed`. This causes problems
-         * when detecting if a model has changed. Bookshelf detects changed attributes too early.
-         * So we have to manually remove invalid model attributes from this object.
-         *
-         * e.g. if you pass `tag.parent` into the model layer, but the value has not changed,
-         * the attribute (`tag.parent`) is still kept in the `changed` object. This is wrong.
-         *
-         * TLDR; only keep valid model attributes in the changed object
-         */
-        this.changed = _.pick(this.changed, Object.keys(this.attributes));
     },
 
     /**