Clean up "plunder_lost_by_nation" statistic

[Goosius1]

Describe the bug

• With current code, the indicated stat is recorded but not displayed
• It is not displayed, because in SW2.0.0 if a besieger were to invade before plundering, this would throw the stat off, as they would be plundering their own nation
• Consider what to do with the code:
  • The recording code could just be removed
  • Or it could be fixed, with some metadata added to the siege object to fix
  • Additionally other useful stats could be added to the war history, which is now in a nice hover-box
    • It could be worthwhile asking servers what stats they would like before proceeding.