[Feature][Frontend]: Deprecate --enable-reasoning (#17452)

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

vllm/engine/llm_engine.py

@@ -2096,7 +2096,7 @@ class LLMEngine:
             guided_decoding.backend = guided_decoding.backend or \
                 self.decoding_config.backend
 
-            if self.decoding_config.reasoning_backend is not None:
+            if self.decoding_config.reasoning_backend:
                 logger.debug("Building with reasoning backend %s",
                              self.decoding_config.reasoning_backend)