Remove forgotten deprecated parse_as_dict argument in LightningCLI test

[mauvilsa]

What does this PR do?

Removes parse_as_dict=False from a LightningCLI test. This parameter was deprecated long ago, and the lightning code was already adapted. But one use remained by mistake. Noted only now while running tests with a version of jsonargparse that no longer accepts this parameter.

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📚 Documentation preview 📚: https://pytorch-lightning--21159.org.readthedocs.build/en/21159/