Remove demo catalog

[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: StaticInputV2 and GeneratedInputV2 missing in paddle.v2.layer #2349 get_config_arg() and make_get_config_arg() in trainer/config_parser.py produces error in demo python script #2173 Remove old api demo #1725 删除demo目录下和book中重复的例子 #1257 gan demo 报错 , AttributeError: copyFrom  #1282 Error while running demo/semantic_role_labeling/train.sh #1785 似乎是个bug，TypeError: precision_recall_evaluator() got an unexpected keyword argument 'top_k' #2181 demo:recommendation,evaluate error #1552 the data set of traffic_prediction demo has some zero values, how can we deal with it? Can we add some values to it? #1505 Embedding在paddle如何实验的问题 #1138 运行paddle/demo/quick_start/train.sh报错 #1385 sequence tagging 任务预估时报 Segmentation fault #1350 安装matplot问题 #1293 demo/quick_start/api_train.sh报错 #2279. 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.