refactor: Remove double filtering based on score threshold (#3019)

# What does this PR do?
Remove score_threshold based check from `OpenAIVectorStoreMixin`

Closes: https://github.com/meta-llama/llama-stack/issues/3018

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## Test Plan
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llama_stack/providers/utils/memory/openai_vector_store_mixin.py

@@ -433,10 +433,6 @@ class OpenAIVectorStoreMixin(ABC):
             # Convert response to OpenAI format
             data = []
             for chunk, score in zip(response.chunks, response.scores, strict=False):
-                # Apply score based filtering
-                if score < score_threshold:
-                    continue
-
                 # Apply filters if provided
                 if filters:
                     # Simple metadata filtering