Set global_http_status from global_type even if it is already set

Now that the site yaml import sets `global_type` only, it is the source
of truth, and so if a global status is removed from the site yaml the
change of `global_type` to `nil` should also set `global_http_status` to `nil`,
rather than leaving it as the original value. If it had been a redirect,
the `global_new_url` would have been removed by the import so the redirect
would error with the current behaviour of Bouncer (still using the old
column).