Drop find_prefixed_util

[oech3]

I guess it was used only for coreutils before #10261

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/follow-name (passes in this run but fails in the 'main' branch)

[codspeed]

CodSpeed Performance Report

Merging this PR will improve performance by 3.78%

_{Comparing oech3:patch-2 (4f9aad9) with main (d72d6f7)}

Summary

⚡ 1 improved benchmark
✅ 281 untouched benchmarks
⏩ 38 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Memory	dd_copy_default	130.8 KB	126 KB	+3.78%

Footnotes

1. 38 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. ↩

[cakebaker]

Thanks!