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GH-45075: [C++] Remove result_internal.h (#45066)

### Rationale for this change

The result_internal.h file contains only the definition of `ASSIGN_OR_RAISE` and thus can be removed because `ARROW_ASSIGN_OR_RAISE` is preferred over `ASSIGN_OR_RAISE`.

### What changes are included in this PR?

Remove result_internal.h and use `ARROW_ASSIGN_OR_RAISE` to replace `ASSIGN_OR_RAISE`.

### Are these changes tested?

Pass CI.

### Are there any user-facing changes?

No.
* GitHub Issue: #45075

Authored-by: Gang Wu <ustcwg@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>