Remove override to Jekyll::Document#respond_to? (#7695)

Merge pull request 7695

lib/jekyll/document.rb

@@ -392,12 +392,6 @@ module Jekyll
       @related_posts ||= Jekyll::RelatedPosts.new(self).build
     end
 
-    # Override of normal respond_to? to match method_missing's logic for
-    # looking in @data.
-    def respond_to?(method, include_private = false)
-      data.key?(method.to_s) || super
-    end
-
     # Override of method_missing to check in @data for the key.
     def method_missing(method, *args, &blck)
       if data.key?(method.to_s)