Remove the demand update feature veto. It's virtually unused, flawed

Remove the demand update feature veto.  It's virtually unused, flawed
by design (by voting last players can tactically vote no and thus
build up veto rights), and its implementation is buggy:
(update_missed): Remove.
(zdone): Don't show it.
(demand_check): Remove veto check.
(natstr, cou_ca): Remove member nat_missed and its selector.
(zdone): Don't clear and don't show it.
(update_removewants): Don't increment it.  This was buggy anyway; it
incremented even on non-demand updates.
(prnat): Don't show it, remove key 'U'.
(docountry): Don't change it, deprecate the now useless key 'U'.