Remove link of PDF download

Because legacy.gitbook.com has been move to the new gitbook.com, PDF export feature is not supported anymore in the free plan.

Close #227

README.md

@@ -19,9 +19,7 @@ Kubernetes 发展非常迅速，已经成为容器编排领域的领导者。Kub
 - 中文：
   - Gitbook: [kubernetes.feisky.xyz](https://kubernetes.feisky.xyz/)
   - Github: [github.com/feiskyer/kubernetes-handbook](https://github.com/feiskyer/kubernetes-handbook/blob/master/SUMMARY.md)
-  - InfoQ: [www.infoq.com/cn/minibooks/Kubernetes-handbook](http://www.infoq.com/cn/minibooks/Kubernetes-handbook)
 - English: [kubernetes.feisky.xyz/v/en](https://kubernetes.feisky.xyz/v/en/)
-- PDF 电子书：点击 [这里](https://legacy.gitbook.com/download/pdf/book/feisky/kubernetes) 下载
 
 ## 项目源码