Remove AsElementAttributeSerializer

The JSON rendering currently is based on inspecting Attributes in Configuration(Builder) classes, and it's also possible to plug-in an AttributeSerializer that will emit the Attribute as a JSON Object rather than a field (property name/value pair) inside a JSON object.

Sometimes it's not always clear what will be the rendered JSON when attributes can be objects and vice-versa, so JSON objects should be generated from Configuration objects and JSON fields should be generated from the Configuration's Attributes.