unlinks Ruby on Rails Tutorial [skip ci]

I added myself links to the Ruby on Rails Tutorial years ago, after
meeting Michael in a RailsConf.

By then, the book was a commercial product, but had a free online
version and I thought it could be a good resource complementing the
official documentation.

Nowadays, only a few sample chapters are available for free, and I
don't consider these links to be fair with the rest of commercial
books about Rails anymore.

README.md

@@ -80,7 +80,6 @@ and may also be used independently outside Rails.
     * [Getting Started with Rails](https://guides.rubyonrails.org/getting_started.html)
     * [Ruby on Rails Guides](https://guides.rubyonrails.org)
     * [The API Documentation](https://api.rubyonrails.org)
-    * [Ruby on Rails Tutorial](https://www.railstutorial.org/book)
 
 ## Contributing
 

guides/source/getting_started.md

@@ -2058,7 +2058,6 @@ getting up and running with Rails, feel free to consult these support
 resources:
 
 * The [Ruby on Rails Guides](index.html)
-* The [Ruby on Rails Tutorial](https://www.railstutorial.org/book)
 * The [Ruby on Rails mailing list](https://groups.google.com/group/rubyonrails-talk)
 * The [#rubyonrails](irc://irc.freenode.net/#rubyonrails) channel on irc.freenode.net
 

railties/RDOC_MAIN.rdoc

@@ -80,7 +80,6 @@ and may also be used independently outside \Rails.
    * {Getting Started with \Rails}[https://guides.rubyonrails.org/getting_started.html].
    * {Ruby on \Rails Guides}[https://guides.rubyonrails.org].
    * {The API Documentation}[https://api.rubyonrails.org].
-   * {Ruby on \Rails Tutorial}[https://www.railstutorial.org/book].
 
 == Contributing