[Bug]: unnecessary auto-increment calculations in the tokens statistics of the chat model (#2969)

### What problem does this PR solve?

the details is shown in
https://github.com/infiniflow/ragflow/issues/2968

### Type of change

- [X] Bug Fix (non-breaking change which fixes an issue)

---------

Co-authored-by: Kevin Hu <kevinhu.sh@gmail.com>

rag/llm/chat_model.py

@@ -68,7 +68,6 @@ class Base(ABC):
                     resp.choices[0].delta.content = ""
                 ans += resp.choices[0].delta.content
 
-                total_tokens += 1
                 if not hasattr(resp, "usage") or not resp.usage:
                     total_tokens = (
                                 total_tokens