Remove unused Struct (#522)

Author: ima1zumi

lib/irb/init.rb

@@ -421,8 +421,6 @@ def IRB.load_modules
     end
   end
 
-
-  DefaultEncodings = Struct.new(:external, :internal)
   class << IRB
     private
     def set_encoding(extern, intern = nil, override: true)