Remove Redundant None Check for vector_similarity_weight (infiniflow#4037)

### What problem does this PR solve?
The removed if statement is unnecessary and adds cognitive load for
readers.
The original code:
```
vector_similarity_weight = req.get("vector_similarity_weight", 0.3)
if vector_similarity_weight is None:
    vector_similarity_weight = 0.3
```
has been simplified to:
```
vector_similarity_weight = req.get("vector_similarity_weight", 0.3)
```

### Type of change
- [x] Refactoring

Author: learnerLj

api/apps/dialog_app.py

@@ -41,8 +41,6 @@ def set_dialog():
         req["rerank_id"] = ""
     similarity_threshold = req.get("similarity_threshold", 0.1)
     vector_similarity_weight = req.get("vector_similarity_weight", 0.3)
-    if vector_similarity_weight is None:
-        vector_similarity_weight = 0.3
     llm_setting = req.get("llm_setting", {})
     default_prompt = {
         "system": """你是一个智能助手，请总结知识库的内容来回答问题，请列举知识库中的数据详细回答。当所有知识库内容都与问题无关时，你的回答必须包括“知识库中未找到您要的答案！”这句话。回答需要考虑聊天历史。