MINOR: [C++] Remove deprecated inclusion (#46672)

### What changes are included in this PR?

The `<ciso646>` inclusion is deprecated in C++20 and doesn't actually serve a purpose, remove it.

### Are these changes tested?

Compilation  passes on CI.

### Are there any user-facing changes?

No.

Authored-by: Antoine Pitrou <[email protected]>
Signed-off-by: Antoine Pitrou <[email protected]>

Author: pitrou

cpp/src/arrow/dataset/test_util_internal.h

@@ -18,7 +18,6 @@
 #pragma once
 
 #include <algorithm>
-#include <ciso646>
 #include <functional>
 #include <memory>
 #include <ostream>