Revert "[ML] Return both modelId and inferenceId (elastic#111490)"

Both `model_id` and `inference_id` were returned from the inference API
to support backwards compatibility while kibana migrates to
`inference_id`.

Now that the migration is complete, we will stop returning `model_id`.

This reverts commit 3b8970c.

server/src/main/java/org/elasticsearch/inference/ModelConfigurations.java

@@ -126,7 +126,6 @@ public XContentBuilder toXContent(XContentBuilder builder, Params params) throws
         if (params.paramAsBoolean(USE_ID_FOR_INDEX, false)) {
             builder.field(INDEX_ONLY_ID_FIELD_NAME, inferenceEntityId);
         } else {
-            builder.field(INDEX_ONLY_ID_FIELD_NAME, inferenceEntityId);
             builder.field(INFERENCE_ID_FIELD_NAME, inferenceEntityId);
         }
         builder.field(TaskType.NAME, taskType.toString());
@@ -143,7 +142,6 @@ public XContentBuilder toFilteredXContent(XContentBuilder builder, Params params
         if (params.paramAsBoolean(USE_ID_FOR_INDEX, false)) {
             builder.field(INDEX_ONLY_ID_FIELD_NAME, inferenceEntityId);
         } else {
-            builder.field(INDEX_ONLY_ID_FIELD_NAME, inferenceEntityId);
             builder.field(INFERENCE_ID_FIELD_NAME, inferenceEntityId);
         }
         builder.field(TaskType.NAME, taskType.toString());