[http-client-java] remove vanilla-related code

[XiaofeiCao]

Back in swagger/autorest time, we have vanilla flavor for some services, e.g. storage. Now that autorest is deprecated, we can cleanup its path.

[github-actions]

🎯 Issue Triage

This is a maintenance task to remove legacy "vanilla flavor" code paths in the Java client emitter that were originally added to support autorest-based services (e.g., Azure Storage). Since autorest is now deprecated, this code can be safely cleaned up.

Category: Task (Code Cleanup)

Area: emitter:client:java

🔍 Analysis

• Affected component: @typespec/http-client-java emitter
• Nature: Technical debt removal — the "vanilla" flavor was a concept from swagger/autorest era where some Azure services (like Storage) had non-standard client generation paths
• Impact: Low risk; removing dead/legacy code paths improves maintainability
• Parent issue: [http-client-java] autorest generator code cleanup #11068
• No duplicates found

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