Cleanup xnnpack_README.md

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✅ You can merge normally! (1 Unrelated Failure)

As of commit 5d421d8 with merge base eecf74f ():

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

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

• Apple / test-demo-ios / mobile-job (ios) (gh) (trunk failure)
  ##[error]The operation was canceled.

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