remove rethinkdb course (#4673)

the course is not online any more

free-courses-en.md

@@ -51,7 +51,6 @@
   * [Flask](#flask)
 * [QB64](#QB64)
 * [R](#r)
-* [RethinkDB](#rethinkdb)
 * [Ruby](#ruby)
 * [Sails.js](#sailsjs)
 * [Scala](#scala)
@@ -536,11 +535,6 @@
 * [R Tutorial For Beginners | Edureka](https://www.youtube.com/watch?v=fDRa82lxzaU) - (Edureka)
 
 
-### RethinkDB
-
-* [RethinkDB: Distributed Databases](https://courses.platzi.com/courses/rethinkdb-databases/)
-
-
 ### Ruby
 
 * [Learn how to program: Ruby](https://www.learnhowtoprogram.com/ruby) - Epicodus Inc.