Fix wrong GROUP_BY on performance stats (bug 1010728)

See https://bugzilla.mozilla.org/show_bug.cgi?id=1010728

Note: the problem was only the missing `user_id` from the GROUP_BY clause (introduced in https://github.com/mozilla/olympia/commit/791b698efb38d450612e0953786b4713adad9f2c#diff-3aa796a7b0957c2c09bdd854786cf8b9L101), but I didn't see any good reason to do a JOIN on users, while only selecting user_id at the end, this is why I also removed the JOIN.