the request object manages `env`

remove the setter.  The request object manages the env hash, so any
mutations need to go through it

actionpack/lib/action_controller/metal.rb

@@ -116,8 +116,6 @@ def build_middleware(klass, args, block)
   class Metal < AbstractController::Base
     abstract!
 
-    attr_internal_writer :env
-
     def env
       @_request.env
     end