Stop checking for unet/model.onnx when a model_index.json is detected (#4132)

## What type of PR is this? (check all applicable)

- [ ] Refactor
- [ ] Feature
- [x] Bug Fix
- [ ] Optimization
- [ ] Documentation Update
- [ ] Community Node Submission


## Have you discussed this change with the InvokeAI team?
- [x] Yes
- [ ] No, because:

      
## Have you updated all relevant documentation?
- [ ] Yes
- [ ] No


## Description


## Related Tickets & Documents

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## QA Instructions, Screenshots, Recordings

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## Added/updated tests?

- [ ] Yes
- [ ] No : _please replace this line with details on why tests
      have not been included_

## [optional] Are there any post deployment tasks we need to perform?

invokeai/backend/install/model_install_backend.py

@@ -303,7 +303,7 @@ class ModelInstall(object):
 
         with TemporaryDirectory(dir=self.config.models_path) as staging:
             staging = Path(staging)
-            if "model_index.json" in files and "unet/model.onnx" not in files:
+            if "model_index.json" in files:
                 location = self._download_hf_pipeline(repo_id, staging)  # pipeline
             elif "unet/model.onnx" in files:
                 location = self._download_hf_model(repo_id, files, staging)