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GH-44629: [C++][Acero] Use `implicit_ordering` for `asof_join` rather than `require_sequenced_output` (#44616)

### Rationale for this change
Changes in #44083 (GH-41706) unnecessarily sequences batches retrieved from scanner where it only requires the batches to provide index according to implicit input order.

### What changes are included in this PR?
Setting `implicit_ordering` causes existing code to set batch index, which is then available to the `asof_join` node to sequence the batches int input order. This replaces some of #44083 changes.

Some code introduced by #44083 turns out to not be required and has therefore been reverted.

### Are these changes tested?
Existing unit tests still pass.

### Are there any user-facing changes?
Reverts some breaking user-facing changes done by #44083.
* GitHub Issue: #41706
* GitHub Issue: #44629

Authored-by: Enrico Minack <github@enrico.minack.dev>
Signed-off-by: Rossi Sun <zanmato1984@gmail.com>