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GH-40152: [C++] Remove redundant invocation of BatchesFromTable (#40173)

### What changes are included in this PR?
This PR fixes a minor performances bug in TableSorter::SortInternal, which invokes BatchesFromTable to get record batches that have been built already.

### Are these changes tested?

Yes, by existing tests.

### Are there any user-facing changes?

No.

* GitHub Issue: #40152

Authored-by: xin.wang <xin.wang@openpie.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>