mknod: remove windows-related code & flags

[cakebaker]

This PR removes a Windows-related function and many #[cfg(not(windows))] because mknod is a unix-only util.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/du/bind-mount-dir-cycle. tests/du/bind-mount-dir-cycle is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/misc/stdbuf (passes in this run but fails in the 'main' branch)