NXP backend: Remove residual onnx references

[MartinPavella]

Summary

This PR removes old references to ONNX which were left over from the rapid development of the early stages.

Test plan

No new features are added. Correct function is tested by all the existing tests.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/14457

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 1 New Failure, 1 Unrelated Failure

As of commit c45fd6e with merge base 6a238e3 ():

NEW FAILURE - The following job has failed:

• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 3208e124e1cb19ee2db095298055eb36ed0292aa196884d607b35fb9a7b00ac6 /exec failed with exit code 1

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

• pull / unittest-arm-backend-with-no-fvp (test_pytest_models) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

This comment was automatically generated by Dr. CI and updates every 15 minutes.

[MartinPavella]

@pytorchbot label "module: nxp" "release notes: nxp"

[mergennachin]

Nice!

[MartinPavella]

The failing test pull / test-samsung-models-linux / linux-job (pull_request) fails on the command mkdir -p /tmp/exynos_ai_lite_core, which seems unrelated to this PR.
All other failing tests seem unrelated to this PR, and are also failing on main.