Remove ActionView::Helpers::CacheHelper#fragment_cache_key

Introduced in e56c635427, `CacheHelper#fragment_cache_key` is a duplicate of `ActionController::Caching::Fragments#fragment_cache_key`.

We now require the view to provide this method on its own (as with `view_cache_dependencies`); `ActionController::Caching::Fragments` exports its version as a `helper_method`.

actionpack/lib/action_controller/caching/fragments.rb

@@ -14,6 +14,12 @@ module ActionController
     #
     #   expire_fragment('name_of_cache')
     module Fragments
+      extend ActiveSupport::Concern
+
+      included do
+        helper_method :fragment_cache_key if respond_to?(:helper_method)
+      end
+
       # Given a key (as described in +expire_fragment+), returns
       # a key suitable for use in reading, writing, or expiring a
       # cached fragment. All keys are prefixed with <tt>views/</tt> and uses

actionview/lib/action_view/helpers/cache_helper.rb

@@ -216,14 +216,6 @@ module ActionView
         end
       end
 
-      # Given a key (as described in ActionController::Caching::Fragments.expire_fragment),
-      # returns a key suitable for use in reading, writing, or expiring a
-      # cached fragment. All keys are prefixed with <tt>views/</tt> and uses
-      # ActiveSupport::Cache.expand_cache_key for the expansion.
-      def fragment_cache_key(key)
-        ActiveSupport::Cache.expand_cache_key(key.is_a?(Hash) ? url_for(key).split("://").last : key, :views)
-      end
-
     private
 
       def fragment_name_with_digest(name, virtual_path) #:nodoc:

actionview/test/template/render_test.rb

@@ -9,6 +9,10 @@ module RenderTestCases
     @assigns = { :secret => 'in the sauce' }
     @view = Class.new(ActionView::Base) do
       def view_cache_dependencies; end
+
+      def fragment_cache_key(key)
+        ActiveSupport::Cache.expand_cache_key(key, :views)
+      end
     end.new(paths, @assigns)
 
     @controller_view = TestController.new.view_context