ios: drop deprecated MPS export step from both iOS scripts

[rascani]

Summary

The iOS demo's testV3WithMPSBackend was removed in meta-pytorch/executorch-examples (#238), since the MPS backend is deprecated for removal in ExecuTorch 1.4 (#18425) and the test had been intermittently failing on the macos-14-xlarge runner pool on what looks like fp16 numerical drift rather than an executorch regression — the MPS runtime in pytorch/executorch hasn't materially changed since the deprecation.

Remove the matching mps_example.py export step from the two iOS demo build scripts (the CI variant .ci/scripts/test_ios_ci.sh and the developer-facing scripts/test_ios.sh) so neither produces an .pte that nothing consumes. Out of scope here: the demo app's MPS enum case and mv3_mps_float16.pte artifact in the executorch-examples build workflow — those want their own follow-up cleanup once MPS is fully removed in 1.4.

Authored with Claude Code.

Test plan

CI

[pytorch-bot]

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❌ 1 Awaiting Approval, 3 New Failures, 1 Pending, 7 Unrelated Failures

As of commit e0b6adb with merge base 08b2987 ():

AWAITING APPROVAL - The following workflow needs approval before CI can run:

• periodic (gh)

NEW FAILURES - The following jobs have failed:

• pull / test-lora-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 3dd0e0117c30c739437f9b6548b138798496aac34c7e72a4dd4d69a00eda2858 /exec failed with exit code 1
• pull / test-lora-multimethod-linux / linux-job (gh)
  RuntimeError: Command docker exec -t d16fda4aec7af8fe03a8448c6cc1f9f12341eff1e7bd868e8e81b9a8963e5f3a /exec failed with exit code 1
• trunk / test-qnn-optimum-model (fp32, pvt) / linux-job (gh)
  RuntimeError: Command docker exec -t 0eac66d123c712cfc03d99db67af0e80551b74abd536b666568f2c24a89ff4c5 /exec failed with exit code 9

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

• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

BROKEN TRUNK - The following jobs failed but was present on the merge base:

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

• pull / unittest / linux / linux-job (gh) (trunk failure)
  examples/models/llama/tests/test_static_attention.py::StaticAttentionTest::test_within_transformer
• pull / unittest / macos / macos-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest-editable / linux / linux-job (gh) (trunk failure)
  examples/models/llama/tests/test_static_attention.py::StaticAttentionTest::test_within_transformer
• pull / unittest-editable / macos / macos-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• trunk / unittest-release / linux / linux-job (gh) (trunk failure)
  examples/models/llama/tests/test_static_attention.py::StaticAttentionTest::test_within_transformer
• trunk / unittest-release / macos / macos-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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