Remove unused Journey code

- `VERSION` shouldn't be there anymore since Journey is technically part
of Action Dispatch now (and thus Action Pack, and follows the normal
Rails versioning scheme)
- `backwards.rb` was only in the file tree because early in the history
  or Journey (back in 2011!), it was moved from under the Rack namespace, to its own
namespace, Journey! This file is no longer required, and is assigning
constants that are no longer needed.

actionpack/lib/action_dispatch/journey/backwards.rb

@@ -1,5 +0,0 @@
-module Rack # :nodoc:
-  Mount = ActionDispatch::Journey::Router
-  Mount::RouteSet = ActionDispatch::Journey::Router
-  Mount::RegexpWithNamedGroups = ActionDispatch::Journey::Path::Pattern
-end

actionpack/lib/action_dispatch/journey/router.rb

@@ -16,9 +16,6 @@ module ActionDispatch
       class RoutingError < ::StandardError # :nodoc:
       end
 
-      # :nodoc:
-      VERSION = '2.0.0'
-
       attr_accessor :routes
 
       def initialize(routes)