fix(mining-app): 删除 heartbeat_service 中未声明的 _apiBaseUrl 赋值

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/mining-app/lib/core/telemetry/presence/heartbeat_service.dart

@@ -49,7 +49,6 @@ class HeartbeatService {
   }) {
     if (_isInitialized) return;
 
-    _apiBaseUrl = apiBaseUrl;
     _config = config ?? PresenceConfig.defaultConfig;
     this.getInstallId = getInstallId;
     this.getUserId = getUserId;