Even though tables.sql names them un_user_id and un_user_ip, they may be named user_id and user_ip in the actual database due to aliasing in MWLBFactory. Or they may not, see T233221 for a similar case.
At least this case is much less complicated, as the indexes were present since the table was created. So there are no update.php patches that might have resulted in both names existing.
After examining queries against user_newtalk, there does not seem to be anything referring to these indexes by name in core or extensions/skins in Gerrit.