After we strip out duplicate entries from 'routers', don't use 'rl'.

We've got to make sure that every single subsequent calculation in
dirserv_generate_networkstatus_vote_obj() are based on the list of
routerinfo_t *after* we've removed possible duplicates, not before.
Fortunately, none of the functions that were taking a routerlist_t
as an argument were actually using any fields other than this list
of routers.

Resolves issue 18318.DG3.