pipes.rs: simplify, fix useless Result conversion

[oech3]

Any splice error is discarded. So no benefit for conversion.
Closes #12419 .
Fix one of a bool issue seen #12365 .

[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/tail/tail-n0f (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/seq/seq-epipe is now being skipped but was previously passing.
Congrats! The gnu test tests/cut/cut-huge-range is now passing!

[codspeed-hq]

Merging this PR will not alter performance

✅ 319 untouched benchmarks
⏩ 46 skipped benchmarks¹

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_{Comparing oech3:useless-conversion (ef8429c) with main (bd908f0)}

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. ↩

[xtqqczze]

@oech3 Please edit the PR description to unlink #12419 #12423 #12365.

[oech3]

No. You are wrong.