Delete unreachable default clause.

[stereotype441]

The Dart analyzer will soon be changed so that if the default clause of a switch statement is determined to be unreachable by the exhaustiveness checker, a new warning of type
unreachable_switch_default will be issued. This parallels the behavior of the existing unreachable_switch_case warning, which is issued whenever a case clause of a switch statement is determined to be unreachable. For details see dart-lang/sdk#54575.

This PR deletes an unreachable default clause from dart_style now, to avoid a spurious warning when the analyzer change lands.

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