remove duplicated frozen_string_literal magic comment

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@53151 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/mkmf.rb

@@ -1,5 +1,4 @@
 # -*- coding: us-ascii -*-
-# frozen_string_literal: false
 # frozen-string-literal: false
 # module to create Makefile for extension modules
 # invoke like: ruby -r mkmf extconf.rb

lib/shellwords.rb

@@ -1,4 +1,3 @@
-# frozen_string_literal: false
 # frozen-string-literal: true
 ##
 # == Manipulates strings like the UNIX Bourne shell