[sim-swap-subscriptions]: Remove allOf for sinkCredential in SubscriptionRequest

[dfischer-tech]

Problem description
Currently, the sinkCredential has a not needed allOf here in the request.
The code-generator it will produce unexpected behaviours having the description inside the allOf

        sinkCredential:
          allOf:
            - description: A sink credential provides authentication or authorization information necessary to enable delivery of events to a target.
            - $ref: "#/components/schemas/SinkCredential"

Expected behavior
Remove the allOf and move the description to the SinkCredential-component

        sinkCredential:
            - $ref: "#/components/schemas/SinkCredential"
<....>

    SinkCredential:
      description: A sink credential provides authentication or authorization information necessary to enable delivery of events to a target.
      type: object
      properties:
        credentialType:
          type: string
          enum:
            - PLAIN
            - ACCESSTOKEN
            - REFRESHTOKEN
          description: "The type of the credential."
      discriminator:
        propertyName: credentialType
        mapping:
          PLAIN: "#/components/schemas/PlainCredential"
          ACCESSTOKEN: "#/components/schemas/AccessTokenCredential"
          REFRESHTOKEN: "#/components/schemas/RefreshTokenCredential"
      required:
        - credentialType