Revert "unexpand: use buffered read"

[sylvestre]

Reverts #10798

[github-actions]

GNU testsuite comparison:

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

[codspeed-hq]

Merging this PR will degrade performance by 39.23%

❌ 2 regressed benchmarks
✅ 282 untouched benchmarks
⏩ 38 skipped benchmarks¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	unexpand_large_file[10]	382.9 ms	630.1 ms	-39.23%
❌	Simulation	unexpand_many_lines[100000]	182.7 ms	300.6 ms	-39.22%

---

_{Comparing revert-10798-unexpand_buffer (5a2e574) with main (4c1536b)}

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. ↩