ARGV: remove unused 'usage' method

Also remove related helper method `Homebrew.help_s`.

Library/Homebrew/cmd/help.rb

@@ -58,10 +58,6 @@ module Homebrew
     exit 0
   end
 
-  def help_s
-    HOMEBREW_HELP
-  end
-
   private
 
   def help_for_command(cmd)

Library/Homebrew/extend/ARGV.rb

@@ -198,11 +198,6 @@ module HomebrewArgvExtension
     options_only.any? { |arg| arg.scan("-").size == 1 && arg.include?(char) }
   end
 
-  def usage
-    require "cmd/help"
-    Homebrew.help_s
-  end
-
   def cc
     value "cc"
   end