remove FUSE workaround

[oech3]

• cat fast path (splice on linux) does not work for 0 size files #9609 (comment) is very artificial and reporter agreed with removing this.
• This is actually kernel's bug and we need to remove std::io::copy if we actually want to fix it
• GNU cat has same bug for 64 KiB+ input (so not compatible with GNU)
• GNU decided to not support the case with wc -c
• This blocks fix for GnuTests.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/tail/overlay-headers (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/cp/link-heap is now being skipped but was previously passing.
Note: The gnu test tests/printf/printf-surprise is now being skipped but was previously passing.

[oech3]

immediately conflicts with #12651