test(bigframes): remove mean_absolute_scaled_error from arima plus sc…

[TrevorBergeron]

…ore expectations

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request removes the 'mean_absolute_scaled_error' metric from the expected results in the test_arima_plus_score and test_arima_plus_score_series test cases within packages/bigframes/tests/system/small/ml/test_forecasting.py. I have no feedback to provide as there were no review comments.