Cleanup Subscription schema to include only required fields and build upwards again.

## What are the schemas that are affected by the issue
subscription
## What are examples of products that are impacted by the issue
Campaign

The current subscription schema was created as part of the lift off from the private git repository. It was mainly designed for Campaign's use-case by @pferdinand .It has remained stale and most of the related schemas have changed. A suggestion that came up in discussion with @cdegroot-adobe was to clean up the schema upto a level that is currently needed and then build it again on need basis. This issue tracks this initiative.

The issue identifies the subscription schema as the affected area and Campaign as an impacted product. Start by reviewing the current subscription schema and its related schemas; the issue does not define which fields are required or a concrete completion test, so the target scope needs to be established before implementation.

Written by the indexing model from the issue text.