uucore/safe_traversal: remove unnecessary vars

[cakebaker]

This PR does three trivial things:

• it removes a few variables that are used in error cases only
• it removes an unnecessary Windows-related cfg as the feature is Linux-only
• it adds the usual file header

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/misc/stdbuf (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/du/long-from-unreadable is no longer failing!