Psych and SafeYAML was loaded Gem.load_yml, these changes is unnecessary

Merged: https://github.com/ruby/ruby/pull/13312

lib/rubygems/package.rb

@@ -219,7 +219,6 @@ class Gem::Package
   # Adds a checksum for each entry in the gem to checksums.yaml.gz.
 
   def add_checksums(tar)
-    require 'psych'
     Gem.load_yaml
 
     checksums_by_algorithm = Hash.new {|h, algorithm| h[algorithm] = {} }
@@ -553,7 +552,6 @@ EOM
   # Reads and loads checksums.yaml.gz from the tar file +gem+
 
   def read_checksums(gem)
-    require_relative 'safe_yaml'
     Gem.load_yaml
 
     @checksums = gem.seek "checksums.yaml.gz" do |entry|

lib/rubygems/safe_yaml.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require 'psych'
-
 module Gem
   ###
   # This module is used for safely loading YAML specs from a gem.  The