chore(multichain-testing): remove starship repo from helm context on …

…`make clean` (#10881)

refs: #10862

## Description

Updates the Makefile so that `make clean` now also removes starship repo from helm context. [Slack thread](https://agoricopco.slack.com/archives/C038E5369FT/p1737493536319709)

### Security Considerations

None

### Scaling Considerations

None

### Documentation Considerations

None

### Testing Considerations

None

### Upgrade Considerations

None

multichain-testing/Makefile

@@ -36,6 +36,7 @@ install:
 	$(CURDIR)/scripts/install.sh --config $(FILE) --name $(NAME) --version $(HELM_VERSION)
 
 delete:
+	-helm repo remove $(HELM_REPO)
 	-helm delete $(NAME)
 
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