Refactor. (#4487)

### What problem does this PR solve?

### Type of change

- [x] Refactoring

rag/utils/doc_store_conn.py

@@ -1,7 +1,6 @@
 from abc import ABC, abstractmethod
 from dataclasses import dataclass
 import numpy as np
-import polars as pl
 
 DEFAULT_MATCH_VECTOR_TOPN = 10
 DEFAULT_MATCH_SPARSE_TOPN = 10
@@ -187,7 +186,7 @@ class DocStoreConnection(ABC):
             knowledgebaseIds: list[str],
             aggFields: list[str] = [],
             rank_feature: dict | None = None
-    ) -> list[dict] | pl.DataFrame:
+    ):
         """
         Search with given conjunctive equivalent filtering condition and return all fields of matched documents
         """