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/client/hud/layers/SettingsModal.ts

@@ -17,7 +17,6 @@ import {
   SetSoundEffectsVolumeEvent,
 } from "../../sound/Sounds";
 import { ShowGraphicsSettingsModalEvent } from "./GraphicsSettingsModal";
-const structureIcon = assetUrl("images/CityIconWhite.svg");
 const cursorPriceIcon = assetUrl("images/CursorPriceIconWhite.svg");
 const darkModeIcon = assetUrl("images/DarkModeIconWhite.svg");
 const emojiIcon = assetUrl("images/EmojiIconWhite.svg");
@@ -133,11 +132,6 @@ export class SettingsModal extends LitElement implements Controller {
     this.requestUpdate();
   }
 
-  private onToggleStructureSpritesButtonClick() {
-    this.userSettings.toggleStructureSprites();
-    this.requestUpdate();
-  }
-
   private onToggleSpecialEffectsButtonClick() {
     this.userSettings.toggleFxLayer();
     this.requestUpdate();
@@ -410,31 +404,6 @@ export class SettingsModal extends LitElement implements Controller {
               </div>
             </button>
 
-            <button
-              class="flex gap-3 items-center w-full text-left p-3 hover:bg-slate-700 rounded-sm text-white transition-colors"
-              @click="${this.onToggleStructureSpritesButtonClick}"
-            >
-              <img
-                src=${structureIcon}
-                alt="structureSprites"
-                width="20"
-                height="20"
-              />
-              <div class="flex-1">
-                <div class="font-medium">
-                  ${translateText("user_setting.structure_sprites_label")}
-                </div>
-                <div class="text-sm text-slate-400">
-                  ${translateText("user_setting.structure_sprites_desc")}
-                </div>
-              </div>
-              <div class="text-sm text-slate-400">
-                ${this.userSettings.structureSprites()
-                  ? translateText("user_setting.on")
-                  : translateText("user_setting.off")}
-              </div>
-            </button>
-
             <button
               class="flex gap-3 items-center w-full text-left p-3 hover:bg-slate-700 rounded-sm text-white transition-colors"
               @click="${this.onToggleAttackingTroopsOverlayButtonClick}"