[ML] Replace useIsDarkTheme() with EUI's colorMode. (#205079)

## 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

- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios
- [x] 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.

Author: walterra

.github/CODEOWNERS

@@ -779,7 +779,6 @@ x-pack/platform/packages/private/ml/inference_integration_flyout @elastic/ml-ui
 x-pack/platform/packages/private/ml/is_defined @elastic/ml-ui
 x-pack/platform/packages/private/ml/is_populated_object @elastic/ml-ui
 x-pack/platform/packages/private/ml/json_schemas @elastic/ml-ui
-x-pack/platform/packages/private/ml/kibana_theme @elastic/ml-ui
 x-pack/platform/packages/private/ml/local_storage @elastic/ml-ui
 x-pack/platform/packages/private/ml/nested_property @elastic/ml-ui
 x-pack/platform/packages/private/ml/number_utils @elastic/ml-ui
@@ -3171,7 +3170,6 @@ x-pack/platform/packages/private/ml/inference_integration_flyout @elastic/ml-ui
 x-pack/platform/packages/private/ml/is_defined @elastic/ml-ui
 x-pack/platform/packages/private/ml/is_populated_object @elastic/ml-ui
 x-pack/platform/packages/private/ml/json_schemas @elastic/ml-ui
-x-pack/platform/packages/private/ml/kibana_theme @elastic/ml-ui
 x-pack/platform/packages/private/ml/local_storage @elastic/ml-ui
 x-pack/platform/packages/private/ml/nested_property @elastic/ml-ui
 x-pack/platform/packages/private/ml/number_utils @elastic/ml-ui

package.json

@@ -665,7 +665,6 @@
     "@kbn/ml-in-memory-table": "link:x-pack/platform/packages/private/ml/in_memory_table",
     "@kbn/ml-is-defined": "link:x-pack/platform/packages/private/ml/is_defined",
     "@kbn/ml-is-populated-object": "link:x-pack/platform/packages/private/ml/is_populated_object",
-    "@kbn/ml-kibana-theme": "link:x-pack/platform/packages/private/ml/kibana_theme",
     "@kbn/ml-local-storage": "link:x-pack/platform/packages/private/ml/local_storage",
     "@kbn/ml-nested-property": "link:x-pack/platform/packages/private/ml/nested_property",
     "@kbn/ml-number-utils": "link:x-pack/platform/packages/private/ml/number_utils",

tsconfig.base.json

@@ -1260,8 +1260,6 @@
       "@kbn/ml-is-defined/*": ["x-pack/platform/packages/private/ml/is_defined/*"],
       "@kbn/ml-is-populated-object": ["x-pack/platform/packages/private/ml/is_populated_object"],
       "@kbn/ml-is-populated-object/*": ["x-pack/platform/packages/private/ml/is_populated_object/*"],
-      "@kbn/ml-kibana-theme": ["x-pack/platform/packages/private/ml/kibana_theme"],
-      "@kbn/ml-kibana-theme/*": ["x-pack/platform/packages/private/ml/kibana_theme/*"],
       "@kbn/ml-local-storage": ["x-pack/platform/packages/private/ml/local_storage"],
       "@kbn/ml-local-storage/*": ["x-pack/platform/packages/private/ml/local_storage/*"],
       "@kbn/ml-nested-property": ["x-pack/platform/packages/private/ml/nested_property"],