[ML] Fix assertion in the Trained Model UI tests (#195566)

## Summary

Removes assertions for success toast messages that were deleted in
#194614


### 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-pack/test/functional/services/ml/trained_models_table.ts

@@ -572,9 +572,6 @@ export function TrainedModelsTableProvider(
 
       await mlCommonUI.waitForRefreshButtonEnabled();
 
-      await mlCommonUI.assertLastToastHeader(
-        `Deployment for "${modelId}" has been started successfully.`
-      );
       await this.waitForModelsToLoad();
 
       await retry.tryForTime(
@@ -600,10 +597,6 @@ export function TrainedModelsTableProvider(
     public async stopDeployment(modelId: string) {
       await this.clickStopDeploymentAction(modelId);
       await mlCommonUI.waitForRefreshButtonEnabled();
-      await mlCommonUI.assertLastToastHeader(
-        `Deployment for "${modelId}" has been stopped successfully.`
-      );
-      await mlCommonUI.waitForRefreshButtonEnabled();
     }
 
     public async openStartDeploymentModal(modelId: string) {