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ARROW-18320: [C++][FlightRPC] Fix improper Status/Result conversion in Flight client (#14859)

I'm not sure you can properly hit the original case described (I couldn't write a failing unit test for this), but the code is clearly wrong so I've changed it to avoid trying to convert OK into a Result.

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>