arrow

GH-34540: [C++] Removed set but unused variable (#34541)



### Rationale for this change

The variable has been left from https://github.com/apache/arrow/commit/144699c4210ac0b75d16395a583671b2874acc9e, simply removed this variable from the code.

It breaks builds when `-Werror` option is used.

### Are these changes tested?

It was successfully built.

* Closes: #34540

Authored-by: Dmitry Kolmakov <ds.kolmakov@gmail.com>
Signed-off-by: Will Jones <willjones127@gmail.com>