User drupal {users} rather than storing name of drupal user table

Per http://forum.civicrm.org/index.php/topic,30309.0.html storing the cms_table doesn't appear to confer any benefit & often trips people up. I am going to look at removing this for d6 users in the first instance.

I checked with Chris B & he couldn't see any reason and as we seem to use more complex setups that most here I guess if there was a potential problem we would spot it