Remove presumed ownership of HeartbeatTimer

threading::Manager is currently never deleted, but if that ever changes,
deleting the stored HeartbeatTimer pointer would be a double-free
since TimerMgr owns it.

src/threading/Manager.h

@@ -136,7 +136,7 @@ private:
 
 	msg_stats_list stats;
 
-	HeartbeatTimer* heartbeat_timer = nullptr;
+	bool heartbeat_timer_running = false;
 };
 
 }