Resolve Ruby and Sapphire's party prompts again each time the menu opens

The prompt a mode shows (Choose a POKéMON., Move to where?...) was kept on the party menu for the rest of the session. PartyMenu.show now clears it, like the menu's other per-open state, so a prompt is resolved once per opening.

src/ui/game3/party_menu.lua

@@ -945,6 +945,7 @@ function PartyMenu.show(sessionParty, moveOverlay, opts)
   end
   opts = opts or {}
   PartyMenu._order = nil
+  PartyMenu._rsPrompt = nil
   if opts.mode == "battle_switch" or opts.mode == "battle_faint" or (opts.mode == "use" and opts.battleOrder) then
     local party0 = sessionParty or (opts.session and opts.session.party)
     local ov0 = moveOverlay or (opts.session and (opts.session.move_overlay or opts.session.moveOverlay))