arpa: garbage collect ns_newmsg/ns_rdata decls

These were brought in by the libbind import, but these functions were
never actually implemented anywhere, only header declarations and symbol
map entries were imported.

Fixes: 046c3635cdb2 ("Bring final version of libbind:")
Fixes: e45764721aed ("Update our stub resolver to final version of ...")
Reported by: ld.lld 16 being --no-undefined-version by default
Sponsored by: https://www.patreon.com/valpackett
Reviewed by: emaste
Pull request: https://github.com/freebsd/freebsd-src/pull/700
Differential Revision: https://reviews.freebsd.org/D38407