Remove inclusion of rubysl gem for rbx on generated Gemfile

From #14026:

    Specific rbx-2 to limit testing on Rubinius 2.x (since there will be
    other versions of Rubinius > 2.x soon).

    Also, as of Rubinius 2.2.5, it is no longer necessary to bundle the
    rubysl gem.

This is what Rails master/4.1 supports, so we don't need to add rubysl to
gemfiles anymore.

railties/lib/rails/generators/app_base.rb

@@ -111,7 +111,6 @@ module Rails
           javascript_gemfile_entry,
           jbuilder_gemfile_entry,
           sdoc_gemfile_entry,
-          platform_dependent_gemfile_entry,
           spring_gemfile_entry,
           @extra_entries].flatten.find_all(&@gem_filter)
       end
@@ -258,14 +257,6 @@ module Rails
         gems
       end
 
-      def platform_dependent_gemfile_entry
-        gems = []
-        if RUBY_ENGINE == 'rbx'
-          gems << GemfileEntry.version('rubysl', nil)
-        end
-        gems
-      end
-
       def jbuilder_gemfile_entry
         comment = 'Build JSON APIs with ease. Read more: https://github.com/rails/jbuilder'
         GemfileEntry.version('jbuilder', '~> 2.0', comment)

railties/test/generators/app_generator_test.rb

@@ -265,13 +265,6 @@ class AppGeneratorTest < Rails::Generators::TestCase
     end
   end
 
-  def test_inclusion_of_platform_dependent_gems
-    run_generator([destination_root])
-    if RUBY_ENGINE == 'rbx'
-      assert_gem 'rubysl'
-    end
-  end
-
   def test_jquery_is_the_default_javascript_library
     run_generator
     assert_file "app/assets/javascripts/application.js" do |contents|