rm: fix unused var warning on non-(unix & windows)

[cakebaker]

This PR contains three trivial changes:

• it fixes an unused var warning on non-(unix & windows) platforms
• it replaces a #[cfg(not(windows))] with #[cfg(unix)]]
• and it groups all handle_writable_directory functions together by moving the path_is_current_or_parent_directory function

[github-actions]

GNU testsuite comparison:

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)