[User Model] Remove retain_previous_owner attribute from "Create User" and "Create Subscription"

[brismithers]

Description

One Line Summary

Remove retain_previous_owner parameter from "Create User" and "Create Subscription" REST calls. This parameter is no longer used.

Details

Motivation

The retain_previous_owner is no longer honored by the backend, it can be removed.

Testing

Manual testing

Using the example app tested flows that drive "Create User" and "Create Subscription" (i.e. Login, create subscription).

Affected code checklist

• Notifications
  • Display
  • Open
  • Push Processing
  • Confirm Deliveries
• Outcomes
• Sessions
• In-App Messaging
• REST API requests
• Public API changes

Checklist

Overview

• I have filled out all REQUIRED sections above
• PR does one thing
  • If it is hard to explain how any codes changes are related to each other then it most likely needs to be more than one PR
• Any Public API changes are explained in the PR details and conform to existing APIs

Testing

• I have included test coverage for these changes, or explained why they are not needed
• All automated tests pass, or I explained why that is not possible
• I have personally tested this on my device, or explained why that is not possible

Final pass

• Code is as readable as possible.
  • Simplify with less code, followed by splitting up code into well named functions and variables, followed by adding comments to the code.
• I have reviewed this PR myself, ensuring it meets each checklist item
  • WIP (Work In Progress) is ok, but explain what is still in progress and what you would like feedback on. Start the PR title with "WIP" to indicate this.

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This change is