xlocale: garbage collect references to strtoq_l/strtouq_l

These were explicitly never implemented (see
lib/libc/locale/DESIGN.xlocale), but were referenced in the
manpage and the symbol map.

Fixes: 3c87aa1d3dc ("Implement xlocale APIs from Darwin")
Reported by: ld.lld 16 being --no-undefined-version by default
Reviewed by: theraven, emaste
Sponsored by: https://www.patreon.com/valpackett
Pull Request: https://github.com/freebsd/freebsd-src/pull/679
Differential Revision: https://reviews.freebsd.org/D38408