Remove unused Structure Sprites setting

The structureSprites toggle was only read by the toggle UIs themselves —
no rendering code ever consulted it. Drops the getter/setter from
UserSettings, both toggle rows (SettingsModal + UserSettingModal), and
the en.json keys. Other-language entries are left for Crowdin to reap.

src/core/game/UserSettings.ts

@@ -158,10 +158,6 @@ export class UserSettings {
     return this.getBool("settings.specialEffects", true);
   }
 
-  structureSprites() {
-    return this.getBool("settings.structureSprites", true);
-  }
-
   darkMode() {
     return this.getBool(DARK_MODE_KEY, false);
   }
@@ -227,10 +223,6 @@ export class UserSettings {
     this.setBool("settings.specialEffects", !this.fxLayer());
   }
 
-  toggleStructureSprites() {
-    this.setBool("settings.structureSprites", !this.structureSprites());
-  }
-
   toggleCursorCostLabel() {
     this.setBool("settings.cursorCostLabel", !this.cursorCostLabel());
   }