Removed extra GROUP BY terms introduced in revision 13603, pending review, following...

Removed extra GROUP BY terms introduced in revision 13603, pending review, following a total DoS of enwiki due to the slow Special:Listusers query. Changing from "GROUP BY user_name" to "GROUP BY user_name,user_id" increaed the time for a LIMIT 50,50 query from 0.01 seconds to 33 seconds.