libdns: Avoid using compound literals (8).

* dirmngr/dns.h (dns_quietinit): Remove.
(dns_hints_i_new): Remove.

Even before our change, dns_quietinit was questionable macro; There
was no place in dns.c which requires overrides in initializer list.
Only redundant zero were.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>