[Model] Remove duplicated message check in Mistral chat completion request (#15069)

Signed-off-by: Brayden Zhong <b8zhong@uwaterloo.ca>

vllm/transformers_utils/tokenizers/mistral.py

@@ -143,10 +143,6 @@ def make_mistral_chat_completion_request(
     if last_message["role"] == "assistant":
         last_message["prefix"] = True
 
-        last_message = cast(Dict[str, Any], messages[-1])
-        if last_message["role"] == "assistant":
-            last_message["prefix"] = True
-
     # mistral-common requires AssistantMessage content to be string [1].
     #
     # [1]: https://github.com/mistralai/mistral-common/blob/f4a06998b75ed78bbf5aaf569590b772ea26c9f6/src/mistral_common/protocol/instruct/messages.py#L80