Remove C# client-name decorators reproduced by deterministic generator naming

[weikanglim]

Parent tracking issue: Azure/azure-sdk-for-net#61941

After each deterministic naming change ships in the production .NET management generator, remove only C# @@clientName decorators that the generator reproduces exactly.

Required workflow

1. Batch changes by affected TypeSpec project.
2. Format and compile every affected project.
3. Run repository-native tsv validation.
4. Regenerate parent and candidate SDK packages using the exact TypeSpec project directory as LocalSpecRepo.
5. Require byte-identical generated source and public APIs.
6. Restore every decorator that changes generated output.
7. Never modify generated SDK source manually.

"Lines reduced" means physical lines occupied by matching decorator statements; comments, imports, and surrounding blank lines are excluded.