[Feature][Frontend]: Deprecate --enable-reasoning

Signed-off-by: chaunceyjiang <chaunceyjiang@gmail.com>

Author: chaunceyjiang

vllm/model_executor/guided_decoding/__init__.py

@@ -103,7 +103,7 @@ async def get_guided_decoding_logits_processor(
         reasoning_backend: str | None = None) -> LogitsProcessor | None:
 
     reasoner = None
-    if reasoning_backend is not None:
+    if reasoning_backend:
         reasoner_class = ReasoningParserManager.get_reasoning_parser(
             reasoning_backend)
         reasoner = reasoner_class(tokenizer)