delete mxfp8 torch._scaled_mm wrapper

[vkuzo]

Summary:

This is no longer needed after
pytorch/pytorch#148461 lands.

Test Plan:

Reviewers:

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Tasks:

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[vkuzo]

Stack from ghstack (oldest at bottom):

• mx: small speedup with dim0 cast #1980
• mx: remove differentiable casts #1978
• mx: temporarily disable the rceil tests #1977
• mx_formats: move inference to the quantize_ API #1971
• mx_formats: move training to the quantize_ API #1970
• use torch.float8_e8m0fnu in mx_formats #1966
• -> delete mxfp8 torch._scaled_mm wrapper #1965

[pytorch-bot]

🔗 Helpful Links

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❌ 1 New Failure

As of commit 87756f9 with merge base 3766ed7 ():

NEW FAILURE - The following job has failed:

• .github/workflows/float8nocompile_test.yaml (gh)

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