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

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

vllm/config.py

@@ -3225,10 +3225,9 @@ class DecodingConfig:
     in the JSON schema. This is only supported for the `guidance` backend and
     is used to better align its behaviour with `outlines` and `xgrammar`."""
 
-    reasoning_backend: Optional[str] = None
+    reasoning_backend: str = ""
     """Select the reasoning parser depending on the model that you're using.
-    This is used to parse the reasoning content into OpenAI API format.
-    Required for `--enable-reasoning`."""
+    This is used to parse the reasoning content into OpenAI API format."""
 
     def compute_hash(self) -> str:
         """