mx: delete triton_f4_to_bf16 kernel

[vkuzo]

Summary:

This is dead code from early days, test is broken, deleting.

Test Plan:

pytest test/prototype/mx_formats/ -s -x

Reviewers:

Subscribers:

Tasks:

Tags:

[vkuzo]

Stack from ghstack (oldest at bottom):

• [wip] mx: expose a fast path for casting to fp4x2 #2832
• mx: delete use_fp4_custom_triton_dequant_kernel option #2831
• -> mx: delete triton_f4_to_bf16 kernel #2830

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/ao/2830

• 📄 Preview Python docs built from this PR

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✅ You can merge normally! (1 Unrelated Failure)

As of commit 59f2bf7 with merge base b6435f9 ():

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• Run 1xH100 Tests / test (H100, linux.aws.h100, --pre torch torchvision torchaudio fbgemm-gpu-genai --index-url https... / linux-job (gh) (trunk failure)
  test/dtypes/test_affine_quantized_float.py::TestAffineQuantizedFloat8Compile::test_expected_kernels_on_gpu_granularity1_float8_config_version_2

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