[ML] Replace useIsDarkTheme() with EUI's colorMode.

[walterra]

Summary

This PR replaces the useIsDarkTheme() hook with EUI's colorMode. Since this was the last hook in @kbn/ml-kibana-theme left this removes the whole package too.

Note that the hook subscribed to an observable and was able to update the theme in place. EUI's colorMode will only be updated after a page refresh. Since updating the Kibana advanced setting to enable dark mode requires a full page refresh too, I guess it's fair to remove this behavior in favor of this simplification.

In the long run we should aim for getting rid of these checks altogether and rely on dark-mode-aware EUI tokens.

Checklist

• Unit or functional tests were updated or added to match the most common scenarios
• This was checked for breaking HTTP API changes, and any breaking changes have been approved by the breaking-change committee. The release_note:breaking label should be applied in these situations.

[elasticmachine]

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