[V0 Deprecation] Deprecate --disable-frontend-multiprocessing

[sfeng33]

Purpose

• Remove the --disable-frontend-multiprocessing CLI flag from BaseFrontendArgs, API server, and benchmarks
• Clean up test parametrizations that varied over this flag, reducing redundant test variants

Test Plan

pytest tests/distributed/test_pipeline_parallel.py -v -s
pytest tests/entrypoints/instrumentator/test_basic.py -v -s
pytest tests/entrypoints/instrumentator/test_metrics.py -v -s
pytest tests/entrypoints/openai/completion/test_completion_with_prompt_embeds.py -v -s
pytest tests/entrypoints/openai/completion/test_shutdown.py -v -s
pytest tests/v1/entrypoints/openai/test_completion.py -v -s

[gemini-code-assist]

Code Review

This pull request successfully removes the deprecated --disable-frontend-multiprocessing CLI flag. The changes are applied consistently across the codebase, including benchmarks, API servers, and tests. The removal simplifies the configuration and test setup, which is a positive improvement. The code changes are correct and align with the goal of deprecating the V0 flag.