MDL-16061 Revert incorrect fix for "Remove 'username' from the $moodleattributes...

MDL-16061 Revert incorrect fix for "Remove 'username' from the $moodleattributes array"

Merged from MOODLE_18_STABLE.

The fix is wrong, as it breaks auth_db_sync_users.php and
auth_ldap_sync_users.php at least. No new users are added to Moodle, as the
username is missing from the new user info record.

The fix needs to go into update_user_record() in lib/moodlelib.php to make it
skip the 'username' key, as we really need get_userinfo() to return the
username as part of the user info array.