Simplify (and fix) passing of guided decoding backend options (#17008)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

vllm/engine/llm_engine.py

@@ -2091,7 +2091,7 @@ class LLMEngine:
 
             tokenizer = self.get_tokenizer(lora_request=lora_request)
             guided_decoding.backend = guided_decoding.backend or \
-                self.decoding_config.guided_decoding_backend
+                self.decoding_config.backend
 
             if self.decoding_config.reasoning_backend is not None:
                 logger.debug("Building with reasoning backend %s",