rgw/zone: remove duplicated startup logic in RGWSI_Zone

rgw/zone: remove duplicated startup logic in RGWSI_Zone

SiteConfig had already loaded the correct configuration without all of
the crazy search_realm_with_zone() stuff which is now confused by
defaults. remove all of this duplicated logic and rely on SiteConfig

removes functions search_realm_with_zone() create_default_zg() and
init_default_zone() which are no longer used

Fixes: https://tracker.ceph.com/issues/71291
Signed-off-by: Casey Bodley <cbodley@redhat.com>