Clean properties.attributes

In Unomi, there are :

• event properties
• source properties
• target properties
  These 3 set of properties provide plenty of opportunities for data collection & extensions. Therefore, there is no real point of having "attributes" inside the properties fields.

The goal of this ticket is to remove "attributes" from all json schemas that are defining events

For instance:
https://github.com/apache/unomi/blob/master/extensions/json-schema/services/src/main/resources/META-INF/cxs/schemas/items/page/page.properties.attributes.json