unregister api for dataset

Test Plan:

**Type checker and check that the build compiles**

**Unit Tests**

**E2E Tests**

// Screenshots and videos
| Before | After |
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| … | … |

llama_stack/providers/remote/datasetio/huggingface/huggingface.py

@@ -63,6 +63,11 @@ class HuggingfaceDatasetIOImpl(DatasetIO, DatasetsProtocolPrivate):
         )
         self.dataset_infos[dataset_def.identifier] = dataset_def
 
+    async def unregister_dataset(self, dataset_id: str) -> None:
+        key = f"{DATASETS_PREFIX}{dataset_id}"
+        await self.kvstore.delete(key=key)
+        del self.dataset_infos[dataset_id]
+
     async def get_rows_paginated(
         self,
         dataset_id: str,