refactor(mm): remove unused metadata logic, fix tests

- Metadata is merged with the config. We can simplify the MM substantially and remove the handling for metadata.
- Per discussion, we don't have an ETA for frontend implementation of tags, and with the realization that the tags from CivitAI are largely useless, there's no reason to keep tags in the MM right now. When we are ready to implement tags on the frontend, we can refer back to the implementation here and use it if it supports the design.
- Fix all tests.

invokeai/backend/model_manager/metadata/fetch/huggingface.py

@@ -91,7 +91,7 @@ class HuggingFaceMetadataFetch(ModelMetadataFetchBase):
             )
 
         return HuggingFaceMetadata(
-            id=model_info.id, name=name, files=files, api_response=json.dumps(model_info.__dict__)
+            id=model_info.id, name=name, files=files, api_response=json.dumps(model_info.__dict__, default=str)
         )
 
     def from_url(self, url: AnyHttpUrl) -> AnyModelRepoMetadata: