[README] Remove Deploy to Docker Cloud button

In December 2018 Docker Cloud has become part of Docker Hub:
https://blog.docker.com/2018/12/the-new-docker-hub/

Since then the "Deploy to Docker Cloud" button is broken (error 404)
with no alternative for Docker Hub, so the button should be removed.

Docker images are still available at
https://hub.docker.com/r/rssbridge/rss-bridge/

README.md

@@ -85,7 +85,6 @@ Deploy
 Thanks to the community, hosting your own instance of RSS-Bridge is as easy as clicking a button!
 
 [![Deploy on Scalingo](https://cdn.scalingo.com/deploy/button.svg)](https://my.scalingo.com/deploy?source=https://github.com/sebsauvage/rss-bridge)
-[![Deploy to Docker Cloud](https://files.cloud.docker.com/images/deploy-to-dockercloud.svg)](https://cloud.docker.com/stack/deploy/?repo=https://github.com/rss-bridge/rss-bridge)
 [![Deploy to Heroku](https://www.herokucdn.com/deploy/button.svg)](https://heroku.com/deploy)
 
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