Remove deep_clone helper

Deep cloning ast nodes is not needed anymore. Parser ast is immutable by
default and mutations do not need to mutate deep_cloned copies anymore.

lib/mutant.rb

@@ -27,7 +27,6 @@ require 'mutant/node_helpers'
 require 'mutant/singleton_methods'
 require 'mutant/constants'
 require 'mutant/support/method_object'
-require 'mutant/helper'
 require 'mutant/random'
 require 'mutant/mutator'
 require 'mutant/mutation'

lib/mutant/helper.rb

@@ -1,23 +0,0 @@
-module Mutant
-  # Module for support methods
-  #
-  # They would normally be defined on the root namespace.
-  # But it is easier to create the Zombie when there are no
-  # References to the root namespace name within the library.
-  #
-  module Helper
-
-    # Return deep clone of object
-    #
-    # @param [Object] object
-    #
-    # @return [Object] object
-    #
-    # @api private
-    #
-    def self.deep_clone(object)
-      Marshal.load(Marshal.dump(object))
-    end
-
-  end # Helper
-end # Mutant

lib/mutant/mutator.rb

@@ -192,7 +192,7 @@ module Mutant
     # @api private
     #
     def dup_input
-      Helper.deep_clone(input)
+      input.dup
     end
 
   end # Mutator