Remove unnecessary caching

`ActiveSupport::Dependencies.constantize(const_name)` calls
`Reference.new` which is defined as
`ActiveSupport::Dependencies.constantize(const_name)` meaning this call
is already cached and we're doing caching that isn't necessary.

actionpack/lib/action_dispatch/routing/route_set.rb

@@ -1,6 +1,5 @@
 require 'action_dispatch/journey'
 require 'forwardable'
-require 'thread_safe'
 require 'active_support/concern'
 require 'active_support/core_ext/object/to_query'
 require 'active_support/core_ext/hash/slice'
@@ -23,7 +22,6 @@ class RouteSet
       class Dispatcher < Routing::Endpoint
         def initialize(raise_on_name_error)
           @raise_on_name_error = raise_on_name_error
-          @controller_class_names = ThreadSafe::Cache.new
         end
 
         def dispatcher?; true; end
@@ -61,10 +59,8 @@ def controller(params, raise_on_name_error=true)
 
       protected
 
-        attr_reader :controller_class_names
-
         def controller_reference(controller_param)
-          const_name = controller_class_names[controller_param] ||= "#{controller_param.camelize}Controller"
+          const_name = "#{controller_param.camelize}Controller"
           ActiveSupport::Dependencies.constantize(const_name)
         end