[Bug]: @azure-tools/typespec-csharp should generate Azure by default

[lmazuel]

Describe the bug

Given we have @typespec/http-client-csharp now as an explicit package, we don't need flavor anymore to select Azure.

Note that I'm not asking specifically to remove flavor, we can discuss if there is an usage for it. But the default should be Azure if we keep it, and it should silently ignore it (no failure) if we decide to skip it.

Reproduction

Starting the azure-tools emitter with no config at all, should generate Azure.

Checklist

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