unregister for memory banks and remove update API (#458)

The semantics of an Update on resources is very tricky to reason about
especially for memory banks and models. The best way to go forward here
is for the user to unregister and register a new resource. We don't have
a compelling reason to support update APIs.


Tests:
pytest -v -s llama_stack/providers/tests/memory/test_memory.py -m
"chroma" --env CHROMA_HOST=localhost --env CHROMA_PORT=8000

pytest -v -s llama_stack/providers/tests/memory/test_memory.py -m
"pgvector" --env PGVECTOR_DB=postgres --env PGVECTOR_USER=postgres --env
PGVECTOR_PASSWORD=mysecretpassword --env PGVECTOR_HOST=0.0.0.0

$CONDA_PREFIX/bin/pytest -v -s -m "ollama"
llama_stack/providers/tests/inference/test_model_registration.py

---------

Co-authored-by: Dinesh Yeduguru <dineshyv@fb.com>

llama_stack/providers/inline/inference/vllm/vllm.py

@@ -108,6 +108,9 @@ class VLLMInferenceImpl(Inference, ModelsProtocolPrivate):
 
         return VLLMSamplingParams(**kwargs)
 
+    async def unregister_model(self, model_id: str) -> None:
+        pass
+
     async def completion(
         self,
         model_id: str,