Remove the active_model require from action_dispatch.

Since we removed the ActiveModel dependenxy from ActionPack at
166dbaa7 we don't need to require it
anymore.

Closes #7370

actionpack/lib/action_dispatch.rb

@@ -26,7 +26,6 @@
 require 'active_support/core_ext/module/attribute_accessors'
 
 require 'action_pack'
-require 'active_model'
 require 'rack'
 
 module Rack