Remove R interactive tutorial (#2882)

Remove R interactive tutorial from the list as it redirects to Pluralsight instead of CodeSchool

free-programming-interactive-tutorials-en.md

@@ -24,7 +24,6 @@
 * [Ocaml](#ocaml)
 * [PHP](#php)
 * [Python](#python)
-* [R](#r)
 * [Ruby](#ruby)
 * [Scala](#scala)
 * [SQL](#sql)
@@ -186,11 +185,6 @@
 * [Python Koans](https://github.com/gregmalcolm/python_koans)
 
 
-### R
-
-* [Try R](http://tryr.codeschool.com)
-
-
 ### Ruby
 
 * [CodeCademy Ruby](https://www.codecademy.com/learn/ruby)