arrow

GH-39387: [C++] Fix compile warning (#39389)



### Rationale for this change

Fix compile warning:
```bash
In file included from /workspace/arrow/cpp/src/arrow/array/array_base.h:26:
/workspace/arrow/cpp/src/arrow/array/data.h:452:19: warning: unused variable 'buffer_length' [-Wunused-variable]
    const int64_t buffer_length = buffers[i].size / static_cast<int64_t>(sizeof(T));
                  ^
/workspace/arrow/cpp/src/arrow/array/data.h:467:19: warning: unused variable 'buffer_length' [-Wunused-variable]
    const int64_t buffer_length = buffers[i].size / static_cast<int64_t>(sizeof(T));
                  ^
2 warnings generated.
```

### What changes are included in this PR?

### Are these changes tested?

### Are there any user-facing changes?

* Closes: #39387

Authored-by: shibei <shibei.lh@gmail.com>
Signed-off-by: Felipe Oliveira Carvalho <felipekde@gmail.com>