rgw: fix radosgw-admin bucket rm with --purge-objects and --bypass-gc

rgw: fix radosgw-admin bucket rm with --purge-objects and --bypass-gc

Call RGWRados::delete_bucket() from rgw_remove_bucket_bypass_gc()
instead of partial copy of code of RGWRados::delete_bucket().

Fix updating user stats after radosgw-admin bucket rm --purge-objects and --bypass-gc
Due to rgw_user(const std::string& s) was called incorrect version of rgw_bucket_sync_user_stats().

Fixes: http://tracker.ceph.com/issues/22122
Fixes: http://tracker.ceph.com/issues/19959
Signed-off-by: Aleksei Gutikov <aleksey.gutikov@synesis.ru>
(cherry picked from commit db42e385d26ee4d1ef94b900102b705d6a794029)