[rubygems/rubygems] Remove unnecessary `ruby_engine` local variable

Under some case, this variable might not end up being used, in which
case running the script would print unused variable warnings.

https://github.com/rubygems/rubygems/commit/a2d6392ada

lib/bundler/installer/standalone.rb

@@ -12,7 +12,6 @@ module Bundler
       end
       File.open File.join(bundler_path, "setup.rb"), "w" do |file|
         file.puts "require 'rbconfig'"
-        file.puts "ruby_engine = RUBY_ENGINE"
         file.puts "ruby_version = RbConfig::CONFIG[\"ruby_version\"]"
         file.puts reverse_rubygems_kernel_mixin
         paths.each do |path|
@@ -27,7 +26,7 @@ module Bundler
       @specs.map do |spec|
         next if spec.name == "bundler"
         Array(spec.require_paths).map do |path|
-          gem_path(path, spec).sub(version_dir, '#{ruby_engine}/#{ruby_version}')
+          gem_path(path, spec).sub(version_dir, '#{RUBY_ENGINE}/#{ruby_version}')
           # This is a static string intentionally. It's interpolated at a later time.
         end
       end.flatten.compact

spec/bundler/install/gems/standalone_spec.rb

@@ -159,7 +159,7 @@ RSpec.shared_examples "bundle install --standalone" do
     it "generates a bundle/bundler/setup.rb with the proper paths" do
       expected_path = bundled_app("bundle/bundler/setup.rb")
       extension_line = File.read(expected_path).each_line.find {|line| line.include? "/extensions/" }.strip
-      expect(extension_line).to start_with '$:.unshift File.expand_path("#{__dir__}/../#{ruby_engine}/#{ruby_version}/extensions/'
+      expect(extension_line).to start_with '$:.unshift File.expand_path("#{__dir__}/../#{RUBY_ENGINE}/#{ruby_version}/extensions/'
       expect(extension_line).to end_with '/very_simple_binary-1.0")'
     end
   end