Fix TrainDistillTest failure by removing obsolete OfflineArrayRecordIterator mock

[bvandermoon]

Description

Remove the outdated mock of distillation_utils.OfflineArrayRecordIterator in test_main_offline_mode_skips_teacher_loading.

In commit 7329370, offline distillation was refactored to use MaxText's standard input pipeline (Grain) instead of the custom array record reader. This removed OfflineArrayRecordIterator from distillation_utils.py. However, the mock in train_distill_test.py was left in place, resulting in AttributeError failures during test setup. Removing the obsolete mock restores the test suite to a passing state.

FIXES: b/518933738

Tests

CI tests passing now

Checklist

• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
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[ajkv-google]

LGTM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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