Remove caching_allowed? from ActionController::Caching

Where is this used? No other code references to this method and it
isn't being tested anywhere. No tests fail when commented out.

actionpack/lib/action_controller/caching.rb

@@ -82,10 +82,6 @@ module ActionController
       end
     end
 
-    def caching_allowed?
-      request.get? && response.status == 200
-    end
-
     def view_cache_dependencies
       self.class._view_cache_dependencies.map { |dep| instance_exec(&dep) }.compact
     end