tail: remove ?Sized

[oech3]

No description provided.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/seq/seq-epipe. tests/seq/seq-epipe is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/date/resolution (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/symlink (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/unexpand/bounded-memory is now passing!
Skip an intermittent issue tests/pr/bounded-memory (was skipped on 'main', now failing)

[xtqqczze]

why?

[oech3]

for readability.

[cakebaker]

Thanks!