Drop monkey patches that are no longer required

config/initializers/action_dispatch.rb

@@ -1,17 +1,2 @@
 # Stop rails from automatically parsing XML in request bodies
 Rails.configuration.middleware.delete ActionDispatch::ParamsParser
-
-# https://github.com/rails/rails/issues/20710
-module ActionDispatch
-  module Assertions
-    def html_document_with_rss
-      @html_document ||= if @response.content_type == Mime::RSS
-                           Nokogiri::XML::Document.parse(@response.body)
-                         else
-                           html_document_without_rss
-                         end
-    end
-
-    alias_method_chain :html_document, :rss
-  end
-end