[Bug]: Remove network variant default computation from API server.

[jglogan]

I have done the following

• I have searched the existing issues
• If possible, I've reproduced the issue using the 'main' branch of this project

Steps to reproduce

At this point the issue is hypothetical; the API server could select a variant that doesn't match what the network plugin actually selects.

Problem description

At present ContainerService picks a default network variant option value when creating bootstrap information for configuring the user's interfaces. This is incorrect - if a network is created without specifying a variant option, it is the responsibility of the network plugin to determine whether it supports variants (if not, the variant can be nil) and if it does, what value to choose when the creator doesn't provide the option.

Environment

- OS: 26
- Xcode: 26
- Container: main

Code of Conduct

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