dead code in pg_upgrade

While trying to decipher this comment:

        /*
         * Do the old cluster's per-database directories share a directory
         * with a new version-specific tablespace?
         */

I noticed that the condition it's referring to

        if (strlen(old_cluster.tablespace_suffix) == 0)

doesn't appear to have been reachable since support for upgrading from
pre-9.2 was removed in v15 (commit e469f0a).  Before then, this case seems
to have only applied to upgrades from v8.4 or older versions.

I'm planning to commit the attached patch shortly after I double-check that
I'm not missing anything.

-- 
nathan