[ML] Remove key attribute from annotation before indexing. (#30183) (#30187)

The UI adds a `key` attribute to annotation objects to store the letter used for labels in the chart and tables. When editing and saving an annotation that `key` could end up being saved to the annotations index. This isn't necesseary since the `key` attribute is just a dynamic label used within the UI. This fixes it by deleting an eventual `key` attribute from the annotations object before saving it to the index.

x-pack/plugins/ml/server/models/annotation_service/annotation.ts

@@ -86,6 +86,7 @@ export function annotationProvider(
     if (typeof annotation._id !== 'undefined') {
       params.id = annotation._id;
       delete params.body._id;
+      delete params.body.key;
     }
 
     return await callWithRequest('index', params);