Update README.md by removing footer (plotly#2335)

Author: noamross

README.md

@@ -107,7 +107,3 @@ guidelines](https://github.com/plotly/plotly.R/blob/master/CONTRIBUTING.md).
 Included are directions for opening issues, asking questions,
 contributing changes to plotly, and our code of
 conduct.
-
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-
-![<https://ropensci.org>](https://www.ropensci.org/public_images/github_footer.png)