Clear node messages on deletion (#419)

## Summary of the Pull Request

_What is this about?_

## PR Checklist
* [ ] Applies to work item: #xxx
* [ ] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/onefuzz) and sign the CLI.
* [ ] Tests added/passed
* [ ] Requires documentation to be updated
* [ ] I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #xxx

## Info on Pull Request

_What does this include?_

## Validation Steps Performed

_How does someone test & validate?_

src/api-service/__app__/onefuzzlib/pools.py

@@ -318,6 +318,7 @@ class Node(BASE_NODE, ORMMixin):
     def delete(self) -> None:
         NodeTasks.clear_by_machine_id(self.machine_id)
         super().delete()
+        NodeMessage.clear_messages(self.machine_id)
 
 
 class NodeTasks(BASE_NODE_TASK, ORMMixin):