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/controllers/users_controller.rb

@@ -119,7 +119,6 @@ class UsersController < ApplicationController
       disable_categorized_saved_searches disable_tagged_filenames
       disable_cropped_thumbnails disable_mobile_gestures
       enable_safe_mode disable_responsive_mode disable_post_tooltips
-      enable_recommended_posts opt_out_tracking
     ]
 
     permitted_params << :name if context == :create

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

app/views/users/edit.html.erb

@@ -66,7 +66,6 @@
         <%= f.input :disable_post_tooltips, :as => :select, :hint => "Disable advanced tooltips when hovering over thumbnails", :collection => [["No", "false"], ["Yes", "true"]], :include_blank => false %>
         <%= f.input :disable_cropped_thumbnails, :as => :select, :hint => "Disable cropped (square) thumbnails on mobile", :collection => [["No", "false"], ["Yes", "true"]], :include_blank => false %>
         <%= f.input :disable_responsive_mode, :as => :select, :collection => [["No", "false"], ["Yes", "true"]], :include_blank => false, :hint => "Disable alternative layout for mobile and tablet" %>
-        <%= f.input :opt_out_tracking, :as => :select, :collection => [["No", "false"], ["Yes", "true"]], :include_blank => false, :hint => "Opt out of tracking" %>
 
         <%= f.input :favorite_tags, :label => "Frequent tags", :hint => "A list of tags that you use often. They will appear when using the list of Related Tags.", :input_html => { :rows => 5, :data => { :autocomplete => "tag-query" } } %>
         <%= f.input :custom_style, :label => "Custom <a href='https://en.wikipedia.org/wiki/Cascading_Style_Sheets'>CSS</a> style".html_safe, :hint => "CSS rules to apply to the whole site.", :input_html => {:size => "40x5"} %>