Created by: luotao1

We will remove entire demo catalog due to following reasons:

1. We decide not to maintain the demo since it uses old api. Please use book and models to learn PaddlePaddle.
2. There are so many issues related to demo currently: #2349 (closed) #2173 (closed) #1725 (closed) #1257 (closed) #1282 (closed) #1785 (closed) #2181 (closed) #1552 (closed) #1505 (closed) #1138 (closed) #1385 (closed) #1350 (closed) #1293 (closed) #2279 (closed). And users may run demos and create new issues for bugs continuously, which increases the burden of PaddlePaddle.
3. The demo catalog hasn't been updated for a long time, though there are some demos which not have corresponding v2 api version in book and models currently, we can refer to release branch to track them when we want to update into v2 api.