Mop-up for AIX-ectomy: remove now-dead test code.

Tom Lane <tgl@sss.pgh.pa.us>

Commit: f0827b443e6014a9d9fdcdd099603576154a3733
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2024-02-28T19:34:19Z
Releases: 17.0
Mop-up for AIX-ectomy: remove now-dead test code.

Commit 0b16bb877 removed the test query added by commit 79b716cfb,
but not the C-language support function used by that query.  I don't
see any plausible reason why we'd need that function again, so throw
it overboard too.

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