Deletes the private method add_template_helper

The method add_template_helper is private and used only in one place.
I guess its purpose was to remove the noise of module_eval at the cost
of an indirection.

However, Module#include is public since Ruby 2.1, and the indirection
is no longer justified for my taste. The loop in the caller is more
straightforward now.

actionpack/lib/abstract_controller/helpers.rb

@@ -110,7 +110,7 @@ def #{method}(*args, &blk)                     # def current_user(*args, &blk)
       #
       def helper(*args, &block)
         modules_for_helpers(args).each do |mod|
-          add_template_helper(mod)
+          _helpers.include(mod)
         end
 
         _helpers.module_eval(&block) if block_given?
@@ -185,16 +185,6 @@ def define_helpers_module(klass, helpers = nil)
           mod
         end
 
-        # Makes all the (instance) methods in the helper module available to templates
-        # rendered through this controller.
-        #
-        # ==== Parameters
-        # * <tt>module</tt> - The module to include into the current helper module
-        #   for the class
-        def add_template_helper(mod)
-          _helpers.module_eval { include mod }
-        end
-
         def default_helper_module!
           module_name = name.sub(/Controller$/, "")
           module_path = module_name.underscore