[rubygems/rubygems] Remove unnecessary and out of date ruby version check

We already do this check in `setup.rb` itself, which is run earlier.

https://github.com/rubygems/rubygems/commit/160cc3f1c5

lib/rubygems/commands/setup_command.rb

@@ -107,15 +107,6 @@ class Gem::Commands::SetupCommand < Gem::Command
     @verbose = nil
   end
 
-  def check_ruby_version
-    required_version = Gem::Requirement.new ">= 2.6.0"
-
-    unless required_version.satisfied_by? Gem.ruby_version
-      alert_error "Expected Ruby version #{required_version}, is #{Gem.ruby_version}"
-      terminate_interaction 1
-    end
-  end
-
   def defaults_str # :nodoc:
     "--format-executable --document ri --regenerate-binstubs"
   end
@@ -148,8 +139,6 @@ By default, this RubyGems will install gem as:
   def execute
     @verbose = Gem.configuration.really_verbose
 
-    check_ruby_version
-
     require "fileutils"
     if Gem.configuration.really_verbose
       extend FileUtils::Verbose