Revert "[Kernel][UX] Add --linear-backend arg for linear kernel selection" (#39538)

[vllm-agent]

Revert of #39538

Reason: This PR is linked to 1 new CI failure in nightly build #66525:

• LM Eval Qwen3.5 Models (B200): test_gsm8k_correctness[Qwen3.5-397B-A17B-NVFP4-DEP2] — GSM8K accuracy 0.8469 fell below the 0.8500 threshold (expected 0.88, tolerance 0.03). The original PR changed linear kernel selection and NVFP4 quantization tests, which is directly relevant to this NVFP4-quantized model evaluation.

Original PR: #39538 — [Kernel][UX] Add --linear-backend arg for linear kernel selection

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Auto-generated by CI failure analyzer. Please verify this revert is appropriate before merging.

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[gemini-code-assist]

Code Review

This pull request removes the --linear-backend CLI argument and its associated configuration logic from the kernel and engine settings. The kernel selection process has been simplified by removing the filtering mechanism that restricted candidates based on this setting, and tests have been updated to use environment variables for backend overrides instead. I have no feedback to provide.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @vllm-agent.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork