[dart] Fix switch on enums not possible

[kuhnroyal]

All enum instances are const so equals/hashCode is not needed.
Removing this allows to switch/case on enum instances.

Fixes #8307

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CC @swipesight (2018/09) @jaumard (2018/09) @josh-burton (2019/12) @amondnet (2019/12) @sbu-WBT (2020/12) @kuhnroyal (2020/12) @agilob (2020/12)

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