ios: drop deprecated MPS export step from both iOS scripts

The iOS demo's `testV3WithMPSBackend` was removed in
meta-pytorch/executorch-examples (companion change), 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.

Author: rascani

.ci/scripts/test_ios_ci.sh

@@ -47,7 +47,6 @@ say "Exporting Models"
 
 python3 -m examples.portable.scripts.export --model_name="$MODEL_NAME" --segment_alignment=0x4000
 python3 -m examples.apple.coreml.scripts.export --model_name="$MODEL_NAME"
-python3 -m examples.apple.mps.scripts.mps_example --model_name="$MODEL_NAME"
 python3 -m examples.xnnpack.aot_compiler --model_name="$MODEL_NAME" --delegate
 
 mkdir -p "$APP_PATH/Resources/Models/MobileNet/"

scripts/test_ios.sh

@@ -64,7 +64,6 @@ say "Exporting Models"
 
 python3 -m examples.portable.scripts.export --model_name="$MODEL_NAME"
 python3 -m examples.apple.coreml.scripts.export --model_name="$MODEL_NAME"
-python3 -m examples.apple.mps.scripts.mps_example --model_name="$MODEL_NAME"
 python3 -m examples.xnnpack.aot_compiler --model_name="$MODEL_NAME" --delegate
 
 mkdir -p "$APP_PATH/Resources/Models/MobileNet/"