api: remove available_includes dependent on current user.

This is to avoid specifying permissions in multiple disjoint places and
because most API users shouldn't need these fields.

app/models/post_flag.rb

@@ -163,8 +163,6 @@ class PostFlag < ApplicationRecord
   end
 
   def self.available_includes
-    includes_array = [:post]
-    includes_array << :creator if CurrentUser.user.is_moderator?
-    includes_array
+    [:post]
   end
 end