users: remove opt_out_tracking account setting.

This setting doesn't do anything any more. It was only ever used to
disable tracking of some (but not all) data in Mixpanel, then later in
Matomo.

app/models/user.rb

@@ -33,6 +33,8 @@ class User < ApplicationRecord
   # - enable_recent_searches (enabled by 499)
   # - disable_cropped_thumbnails (enabled by 22)
   # - has_saved_searches
+  # - opt_out_tracking
+  # - enable_recommended_posts
   BOOLEAN_ATTRIBUTES = %w(
     is_banned
     has_mail