Fix PetraAI requesting tribute from defeated player.

The requests weren't deleted, both from received and sent diplomacy requests.

Reported by @Obelix at https://wildfiregames.com/forum/topic/96576-petraai-bot-doesnt-recognize-defeat-of-competitor-its-trading-with.
Investigated by: @Langbart
Differential revision: https://code.wildfiregames.com/D4904
Comment by: @phosis
Fixes #6654