Remove wrong namespace comment (onnx#5383)

### Description
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### Motivation and Context
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Somehow the namespace comment which would be generated by tools like
clang-format seems to be left behind

Signed-off-by: Takeshi Watanabe <[email protected]>
Co-authored-by: Xavier Dupré <[email protected]>

onnx/defs/reduction/defs.cc

@@ -168,7 +168,7 @@ The type of the output tensor is integer.)DOC";
       }
     });
   };
-} // namespace ONNX_NAMESPACE
+}
 
 ONNX_OPERATOR_SET_SCHEMA(ArgMax, 13, OpSchema().FillUsing(ArgReduceDocGenerator("max")));