CLEANUP: resolvers: don't close the nameserver socket twice on deinit

CLEANUP: resolvers: don't close the nameserver socket twice on deinit

resolvers_destroy() calls fd_delete() on the UDP socket of each nameserver,
which already closes it, then calls close() on the same FD again. This
second close() fails with EBADF (visible with strace on soft-stop), and
could close an unrelated FD if the number were reused in between. Let's
just drop the redundant close().

This came with resolvers_destroy() in 2.7 (commit e606c84fe "MINOR:
resolvers: resolvers_destroy() deinit and free a resolver"). There is no
need to backport it, though it does not hurt.