Remove named_g_sessionkey as it is unused

It is never assigned after it is declared to be NULL.

bin/named/include/named/globals.h:EXTERN dns_tsigkey_t *named_g_sessionkey    INIT(NULL);
bin/named/server.c:     if (named_g_sessionkey != NULL) {
bin/named/server.c:             dns_tsigkey_detach(&named_g_sessionkey);