Cortex-M: remove extractScalarToInt helpers, use static_cast directly

[rascani]

Summary

Since #16322 changed all Scalar parameters to int64_t, the extractScalarToInt32 and extractScalarToInt helpers in cortex_m_ops_common.h are unnecessary indirection — they called Scalar::to<int64_t>() then cast, but the values are already int64_t.

Replace all call sites with direct static_cast and remove the helpers, the unused Scalar type alias, and the scalar_utils.h include. Also update the quantized_mul Python schema in operators.py to use int instead of Scalar, matching the C++ kernel and operators.yaml.

Additionally remove a duplicate kernel_includes.h include from cortex_m_ops_common.h.

Test plan

pytest backends/cortex_m/test/

[pytorch-bot]

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❌ 5 New Failures, 1 Unrelated Failure

As of commit 79c13af with merge base 119a099 ():

NEW FAILURES - The following jobs have failed:

• pull / test-binary-size-linux-gcc / linux-job (gh)
  RuntimeError: Command docker exec -t 91aa6247c646d0eec97825318568088c282e9795c4107406964655b6baea9bae /exec failed with exit code 1
• pull / test-openvino-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 1814c93dc9abc9fbb905b7a30d968d200a0eeee911e2d9780dc9dde3e039af10 /exec failed with exit code 1
• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 7ebadbbb4ae70e183a14bc783c36dfd17600322b2af952a41709475986a4da3b /exec failed with exit code 1
• pull / test-samsung-quantmodels-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 34c432005eb45cd0c42fed1f942a5278c8aeb2b90a100b00bb1bf6a0099b5a1d /exec failed with exit code 1
• pull / unittest-nxp-neutron / linux-job (gh)
  RuntimeError: Command docker exec -t e6f0f3067c5fcd14037c4a1793197717c04b612068c702323b7bee805ae437f8 /exec failed with exit code 1

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-models-linux (linear, xnnpack-quantization-delegation, linux.2xlarge) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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