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/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"} %>