[geofencing-subscriptions]: Remove "allOf" for sinkCredential in SubscriptionRequest

[maxl2287]

Problem description
Currently the sinkCredential has a not needed allOf here in the request.

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

Commonalities:
https://github.com/camaraproject/Commonalities/blob/main/artifacts/camara-cloudevents/event-subscription-template.yaml#L299-L300

        sinkCredential:
          $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