[Adaptive JetStream] light and dark color schemes are exactly the same

The color schemes used in the app are exactly the same for both light and dark modes:

https://github.com/android/adaptive-apps-samples/blob/main/AdaptiveJetStream/jetstream/src/main/java/com/google/jetstream/presentation/theme/Color.kt

Either dark mode should be supported or it shouldn't and the dark color scheme should be removed.

Start by reading AdaptiveJetStream/jetstream/src/main/java/com/google/jetstream/presentation/theme/Color.kt and compare the light and dark color scheme definitions. Decide whether dark mode should be supported or removed, then update the color scheme accordingly; done means the schemes are no longer identical or the unused dark scheme is gone.

Written by the indexing model from the issue text.