[mlir] Fix -Wunused-variable in TransformInterfaces.h (NFC)

/llvm-project/mlir/include/mlir/Dialect/Transform/IR/TransformInterfaces.h:235:12: error: unused variable 'sameTimestamp' [-Werror,-Wunused-variable]
      bool sameTimestamp =
           ^
/llvm-project/mlir/include/mlir/Dialect/Transform/IR/TransformInterfaces.h:260:12: error: unused variable 'sameTimestamp' [-Werror,-Wunused-variable]
      bool sameTimestamp =
           ^
2 errors generated.