Remove dns_zone_setflag()
This function has never been used since it was added to the source tree by commit 686b27bf back in 1999. As the dns_zoneflg_t type is only defined in lib/dns/zone.c, no function external to that file would be able to use dns_zone_setflag() properly anyway - the DNS_ZONE_SETFLAG() and DNS_ZONE_CLRFLAG() macros should be used instead. Zone options that can be set from outside zone.c are set using dns_zone_setoption().
(cherry picked from commit 314b5362)
Closes #2550 (closed)