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/dmail.rb

@@ -197,8 +197,6 @@ class Dmail < ApplicationRecord
   end
 
   def self.available_includes
-    includes_array = [:owner, :to, :from]
-    includes_array << :moderation_reports if CurrentUser.is_moderator?
-    includes_array
+    [:owner, :to, :from]
   end
 end