[FIX] name_search: change behaviour back to the one before display_name field

As for the name_get, having a different name_search would not affect the display_name field computation. However reusing the name_search for the _search_display_name method would not be efficient (search followed by name_get to just retrieve the ids, also no respect of limit parameter) and in most of the cases would be similar as searching on the _rec_name. To avoid confusion and performance issues, it's better to simply remove search on display_name (and maybe finding a better solution for v9).