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GH-34547: [C++][ORC] Remove deprecated ORC_UNIQUE_PTR (#34548)

### Rationale for this change

`ORC_UNIQUE_PTR` is used to wrap `std::auto_ptr` and `std::unique_ptr` before wide adoption of C++11 in the Apache ORC C++ library. Now the library has adopted C++17 already and this macro has been deprecated.

### What changes are included in this PR?

Replace `ORC_UNIQUE_PTR` with `std::unique_ptr`.

### Are these changes tested?

Make sure all tests pass.

### Are there any user-facing changes?

No.
* Closes: #34547

Authored-by: Gang Wu <ustcwg@gmail.com>
Signed-off-by: Will Jones <willjones127@gmail.com>