Arm backend: Remove unused functions

[Sebastian-Larsson]

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/13093

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❌ 3 New Failures, 1 Unrelated Failure

As of commit 9478186 with merge base 8d0053c ():

NEW FAILURES - The following jobs have failed:

• pull / test-moshi-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 6309fea18284f4ec0e1ac9a446b9451b16fcd55951b441f0ddade9162fd18193 /exec failed with exit code 1
• trunk / test-arm-cortex-m-size-test (bare_metal) / linux-job (gh)
  RuntimeError: Command docker exec -t 613423f164da4dda5e809cb8f70797b203394c2b37427e0abbb484590eeff822 /exec failed with exit code 1
• trunk / test-llama-runner-mac (fp32, coreml) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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

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

• pull / test-openvino-linux / linux-job (gh) (trunk failure)
  AttributeError: '_OpNamespace' 'quantized_decomposed' object has no attribute 'convert_element_type'

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[Sebastian-Larsson]

CI failures are unrelated

[oscarandersson8218]

LGTM!