[ruby/irb] Revert "Memoize helper method instances with Singleton module"

This reverts commit https://github.com/ruby/irb/commit/169a9a2c3097.

https://github.com/ruby/irb/commit/221b0a4928

lib/irb/helper_method/base.rb

@@ -1,10 +1,6 @@
-require "singleton"
-
 module IRB
   module HelperMethod
     class Base
-      include Singleton
-
       class << self
         def description(description = nil)
           @description = description if description

lib/irb/workspace.rb

@@ -179,7 +179,7 @@ EOF
     def self.install_helper_methods
       HelperMethod.helper_methods.each do |name, helper_method_class|
         define_method name do |*args, **opts, &block|
-          helper_method_class.instance.execute(*args, **opts, &block)
+          helper_method_class.new.execute(*args, **opts, &block)
         end unless method_defined?(name)
       end
     end

test/irb/test_helper_method.rb

@@ -97,38 +97,13 @@ module TestIRB
       RUBY
 
       output = run_ruby_file do
-        type "my_helper"
+        type <<~INPUT
+          my_helper
+        INPUT
         type "exit"
       end
 
       assert_include(output, 'Hello from MyHelper')
     end
-
-    def test_helper_method_instances_are_memoized
-      write_ruby <<~RUBY
-        require "irb/helper_method"
-
-        class MyHelper < IRB::HelperMethod::Base
-          description "This is a test helper"
-
-          def execute(val)
-            @val ||= val
-          end
-        end
-
-        IRB::HelperMethod.register(:my_helper, MyHelper)
-
-        binding.irb
-      RUBY
-
-      output = run_ruby_file do
-        type "my_helper(100)"
-        type "my_helper(200)"
-        type "exit"
-      end
-
-      assert_include(output, '=> 100')
-      assert_not_include(output, '=> 200')
-    end
   end
 end