Remove application theme settings from preferences

app/views/profiles/preferences/show.html.haml

@@ -2,13 +2,6 @@
 = render 'profiles/head'
 
 = form_for @user, url: profile_preferences_path, remote: true, method: :put, html: { class: 'row prepend-top-default js-preferences-form' } do |f|
-  .col-lg-3.profile-settings-sidebar
-    %h4.prepend-top-0
-      Application theme
-    %p
-      This setting allows you to customize the appearance of the site, e.g. the sidebar.
-  .col-sm-12
-    %hr
   .col-lg-3.profile-settings-sidebar
     %h4.prepend-top-0
       Syntax highlighting theme

doc/profile/preferences.md

@@ -3,13 +3,6 @@
 Settings in the **Profile > Preferences** page allow the user to customize
 various aspects of the site to their liking.
 
-## Application theme
-
-Changing this setting allows the user to customize the color scheme used for the
-navigation bar on the left side of the screen.
-
-The default is **Charcoal**.
-
 ## Syntax highlighting theme
 
 _GitLab uses the [rouge ruby library][rouge] for syntax highlighting. For a