wc: remove splice_exact

[oech3]

Usage of splice_exact is valid for wc -c because it just discards input. But I want to remove it since it has dangerous interface. See #12315 (comment) .

[codspeed-hq]

Merging this PR will not alter performance

✅ 319 untouched benchmarks
⏩ 46 skipped benchmarks¹

---

_{Comparing oech3:wc-splice-exact (87a9e09) with main (db25551)}

Footnotes

1. 46 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/cut/cut-huge-range is now being skipped but was previously passing.