Arm backend: Remove run-mcu-models job

[AdrianLundell]

Cortex-M testing is now contained in backends/cortex_m/test

@GregoryComer @psiddh

cc @freddan80 @per @zingo @oscarandersson8218 @digantdesai

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15442

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❗ 1 Active SEVs

There are 1 currently active SEVs. If your PR is affected, please view them below:

• Long queue for ROCM runners, also B200 and XPU queueing is observed

❌ 1 Cancelled Job, 2 Pending

As of commit d1cf269 with merge base 3485495 ():

CANCELLED JOB - The following job was cancelled. Please retry:

• trunk / test-models-macos-coreml (mv2) / macos-job (gh)
  ##[error]The operation was canceled.

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