Cleanup nations (Part 3) 🧹 Remove nation strength (#2649)

## Description:

This PR removes the nation strength. Reasoning:

- It is currently unused. The backstory can be found in #2498
- It forces map-makers to do balancing work, which is probably not a
good idea
- It increases map-making work
- It increases nation balancing complexity by a lot (we need to have all
the json files in mind)
- It makes humans avoid map areas completely because there is always
that one, same, strong nation
- The map lead Nikola123 wants to "not deal with the stupid nations and
their balancing"

If the goal of nation strength was to make them feel different I would
suggest a nation personality system. Nations that love to boat, love to
ally, love to nuke, love to fullsend, etc.

Link to a discord discussion about nation strength:


https://discord.com/channels/1359946986937258015/1360078040222142564/1450973197251117218

## Please complete the following:

- [X] I have added screenshots for all UI updates
- [X] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [X] I have added relevant tests to the test directory
- [X] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

## Please put your Discord username so you can be contacted if a bug or
regression is found:

FloPinguin

src/core/configuration/Config.ts

@@ -94,7 +94,6 @@ export interface Config {
   userSettings(): UserSettings;
   playerTeams(): TeamCountConfig;
 
-  useNationStrengthForStartManpower(): boolean;
   startManpower(playerInfo: PlayerInfo): number;
   troopIncreaseRate(player: Player | PlayerView): number;
   goldAdditionRate(player: Player | PlayerView): Gold;