libfetch: cleanups

Apply upstream fix for error messages from FreeBSD: https://cgit.freebsd.org/src/commit/lib/libfetch?id=631b82aca0fd41c8e0d48eebdb9c4e38b7306472

Remove unused variable: https://cgit.freebsd.org/src/commit/lib/libfetch?id=ce700f78f7fb28a252978382a1d0a66d08b6469a