[Device-data-volume-subscriptions]: Remove allOf for sinkCredential in SubscriptionRequest

[ravindrapalaskar17]

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`