Created by: luotao1

Since the document deployment now is using https://github.com/PaddlePaddle/Paddle/blob/4a5dd62b22e8a14b75a572de23beb921ab6d1440/.travis.yml#L56-L59 Thus, we can remove deploy_doc function in scripts/travis/build_doc.sh https://github.com/PaddlePaddle/Paddle/blob/4a5dd62b22e8a14b75a572de23beb921ab6d1440/paddle/scripts/travis/build_doc.sh#L19-L91

And remove gh-pages branch.